Make a api in .net core.

use what is already there for you , .If you check the individual account checkbox then you get ready made code for login, logout , external login with OAuth ,

however , if you want to start from scratch then here are the steps for JWT

https://www.c-sharpcorner.com/article/authentication-and-authorization-in-asp-net-core-web-api-with-json-web-tokens/