Modify the built in token service to include Custom Headers/Body

Hello,

I'd like to modify the currently implemented Auth services in Nebular to include headers of my own, which files should I modify to include simple Username/Password JWT? I couldn't modify the currently EmailPassowrdAuthService to includea simple username and password auth.

I couldn't find the Token body being used even to send to my Auth api end point. Which auth modules should I configure in this case?