Replace Headers and Response

I have been trying to follow the guide for fixing incomplete SP initiated SAML requests. Fixing Incomplete SAML SP Initiated Login, but I am attempting with Google Gsuite which is behaving differently. I am able to extract the relaystate, but not sure if I can inject it in or replace the header or can I create my own Get request method and insert the relaystate and custom rerferer header?

When I look in Fiddler I am seeing that the Referer is still set to IDP.