How to use SAML based SSO in a React based SaaS apps

Question

I am trying to implement SSO using SAML for our SaaS apps ( React + Typescript ) . However, the key issue here is that the SSO is only for selected customers , for which our application has to be protected using customers' IDP. Hence it is NOT possible for me to use the node libraries like passport / passport-saml as in --

passport.use(new SamlStrategy(
  {
    path: 'callback',
    entryPoint: 'IDP endpoint',
    issuer: 'passport-saml',
    cert: 'cert', // cert must be provided
  },
  ....

}

I have to programatically route the authentication to customers' IDP for selected customers.

My question is , for UI resources/pages, do we need to post SAML AuthnRequest OR should I simply route the login UI to customer's IDP endpoint as in --

window.location.replace(IDP_ENDPOINT)

In this way, I am able to redirect to SAML endpoint and able to receive the SAML assertion in my callback Url . However I am not able to pass on the relay state while redirecting the browser.

Any pointer for SSO integration using SAML v2 in typescript environment is highly appreciated.