Finding Authentication for HTTP Request

Question

I'm a bit at the end of my rope here and need help. I'm trying to automate an HTTP request for a site that doesn't have an API. It is hosted on-premises by my company but it is a vended solution with hardly any documentation. I've tried to pass user name and password in just about every way I can think of including basic HTTP, cookies, params, body, etc. I'm also using Postman to see if it helps but I've been unable to crack this. This is the section of the HTML page that references the login fields. Is there anything that stands out? Is there another route I can go to figure this out?

<div id="login_center_section">
    <div id="login_left_border"></div>
    <div id="login_right_border"></div>
    <div id="body_section">
        <div class="content">
            <form method="POST"
                  action="j_security_check"
                  name="loginForm"
                  onsubmit="return validateForm();">
               <div id="authentication_error">
                   <table height="100%">
                       <tr>
                           <td valign="middle">
                               <img src="public/img/warning_icon.gif"
                                    alt="" 
                                    style="margin: 0px 10px 0px 10px;" />
                           </td>
                           <td valign="middle">
                               <span class="errorlabel">Authentication failed. Try another username and password.</span>
                           </td>
                       </tr>
                   </table>
               </div>
               <div id="noDB" 
                    class="smboldblack">
                   <table height="100%">
                       <tr>
                           <td valign="middle">
                               <img src="public/img/warning_icon.gif" 
                                    alt="" 
                                    style="margin: 0px 10px 0px 10px;" />
                           </td>
                           <td valign="middle">
                               <span class="smboldblack">The database connection has failed.  Logins are disabled until the database is restored.</span>
                           </td>
                       </tr>
                   </table>
               </div>
               <div id="input" 
                    style="position: absolute; left: 90px; top: 60px;">
                   <table>
                       <tr id="usr">
                           <td class="lab">
                               <label class="boldblack" 
                                      for="username">
                                   <span style="white-space: nowrap;">Username:</span>
                               </label>
                           </td>
                           <td>
                               <input name="j_username" 
                                      type="text" 
                                      size="22" 
                                      maxlength="128" 
                                      id="username" 
                                      value="" 
                                      autocomplete="off"/>
                           </td>
                           <td id="lbl_username_not_empty_msg" 
                               style="display: none;" 
                               width="100px">
                               <img src="public/img/warning_icon.gif"
                                    style="margin: 0px 10px 0px 10px;"/>
                               <label class="errorlabel">Username cannot be empty.</label>
                           </td>
                       </tr>
                       <tr></tr>
                       <tr></tr>
                       <tr id="pwd" 
                           style="display: none;">
                           <td class="lab">
                               <label id="pwdlbl"
                                      class="boldblack" 
                                      for="password">
                                   <span style="white-space: nowrap;">Password:</span>
                               </label>
                           </td>
                           <td>
                               <input name="j_password" 
                                      type="password" 
                                      size="22" 
                                      id="password" 
                                      value="" 
                                      autocomplete="off"/>
                           </td>
                           <td id="lbl_password_not_empty_msg" 
                               style="display: none;" 
                               width="100px">
                               <img src="public/img/warning_icon.gif" 
                                    style="margin: 0px 10px 0px 10px;"/>
                               <label class="errorlabel">Password cannot be empty.</label>
                           </td>
                       </tr>
                   </table>
               </div>