This sketch app plugin should do the following two things
1) Login to our application using username/password and get a JWT token
2) Send (or Sync) all or the selected screens from the Sketch canvas to our online application
The API implementation is not a part of this job, and I'll provide you with all the details where to send the screens to. API's are ready to use.
If there is another better way to create a plugin then happy to consider that too.