Keycloak give user authentication to another app

Hi, I have 2 applications those access the same nestjs backend. Application A is the portal anyone can log in to that application using a username and password.

Application B is the meeting application that use to meet the users who have signup into portal B.

so there are 2 users X, Y both are registered in the system both can log into application A

Say X log into the portal and generate the meeting link for application B and send it to user Y through the email(through the application)

when X,Y opens the link application B will open and I want to give permission to X, and Y to access the same nestjs backend. can you suggest something?

how can I do it using keycloak