I am creating a webapp and I want to implement login and signup using speckle server and get access token of logged in user.
Impersonating a user or applying a service account pattern is tricky.
It is likely there is a better way to achieve whatever it is you are trying to do.
The graphql api will allow you to invite a user to a server. This invitation will have to be accepted and the email verified by that user.
They may then either:
- authenticate your process as a registered app
- register their own PAT with whatever your app does
[EDIT] my misunderstanding of your question - @teocomi has you covered 
Can I achieve user login and signup on my webapp by using speckle server?
Plenty of resources on how to do that, just look around 
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