Setup IDP initiated SSO with third party vendor

Hello,

I am trying to setup SSO with a third party vendor. They only accept IDP initiated SSO. So when I setup their client in Keycloak, I am having a hard time figure out what the URL I will need to use to be able to authenticate into their application.
Ive following these examples with no luck:

https://groups.google.com/g/keycloak-user/c/s_sVxPGLhCs?pli=1

Can anyone provide some insight?