Access Tokens from Firebase in Forge App


I am creating a custom ui forge app in Jira. I want to access my backend apis that I have stood up. These apis require an access token corresponding to a user that’s all managed within firebase. From my client (that I own), I retrieve a token for that user after providing username/password. In the absence of a username and password, what is the solution to retrieving an access token for a user from firebase?