This identity in 15 session will walk you through how to add a custom grant type for OAuth2 with sample custom mobile grant type.

Write a Custom OAuth 2.0 Grant Type [DOC Link]
https://is.docs.wso2.com/en/latest/references/extend/oauth2/write-a-custom-oauth-2.0-grant-type/#implement-a-new-grant-type

Writing a Custom OAuth2 Grant Type in WSO2 Identity Server
https://dhaurapathirana.medium.com/writing-a-custom-oauth2-grant-type-in-wso2-identity-server-6ab92881fdaf

Access Delegation with OAuth2
https://dhaurapathirana.medium.com/access-delegation-with-oauth2-a30837ce1e83