How to add attributes to SAML Request

Hello guys, dear colleagues,

I have to do my school work and I get to touch with Keycloak. It is really powerful IAM and really customizable but I got problem.
My network is:
WEB - Keycloak - External IdP (with specific SAML Attributes).
I have to do login between Keycloak and External IdP with SAML with specific attributes in SAML Request. Then, after login, user is redirected on my WEB with OIDC.
But I am completely lost with SAML Request. I need to get attributes here but I have no idea how. It is possible through GUI or I need to do some coding? And where specifically? I am pretty new to Java. This example helps me a little but I need SAML.

Thank You for Your help.

Any suggestions on this kind of problem? Thank You