Specify folder to save files to in Google Script

Question

I have a simple form published through Google Apps Scripts that lets someone upload a file anonymously. The script works well, but it will always save to the root of my Google Drive instead of to a specific folder. I'd like it to create a new folder with the person's first name + last name (collected from the form) within a specific folder. I know I need to use the destination FolderID, just not sure how to use it properly.

Here's my existing code:

function uploadFiles(form) {

  try {

var dropbox = form.FirstName + form.LastName;
var folder, folders = DriveApp.getFoldersByName(dropbox);

if (folders.hasNext()) {
  folder = folders.next();
} else {
  folder = DriveApp.createFolder(dropbox);
}

var blob = form.myFile;    
var file = folder.createFile(blob);    
file.setDescription("Uploaded by " + form.Name);

return "Thank you, your file was uploaded successfully!";