Credentials provider: post callback route

[HSorensen]

I am working with next-auth to create a login to an enterprise software (SAP) for our Next.js application.

Where can I find the implementation of the callback ?

<form method="post" action="/api/auth/callback/credentials" >
</form>

As far as I can see the callback expects the post to have a form encoded in the body of the post response.

The SAP has a callback of their login form that I am trying to use for the next-auth credentials provider but the body of the post command is not what the callback/credentials expects.
The issue is the authorize function of the credentials provider does not get invoked.
Any hints would be appreciated.

Thanks