And why do you need to get the access token? if you send the request to backend workflow with Schedule API Workflow from the user account (when the user is logged in an he is the one who call the backend WF), this will use this user access_token.
This is not something you can access, however, you can use a workaround:
Create a backend workflow with detect request header enabled, add API call to this backend workflow inside the API configured to call Discord. This request will send the access_token in the header that you can after, use in another call (but you will need to create another API with selfhandled set to be able to use this call without conflict). Honestly, this is not a good thing.
Instead, you should consider configuring the oauth process manually (full process). [Showcase] Manual OAuth2 Token Integration