Presumably part of what you want to do is to make a rest call and use the result in a subsequent step? If so these links may be of help:
Rest call task | Alfresco Documentation
REST Call Task and People form variables in the request
You can also choose an endpoint for a typeahead. There's a general rest integration guide in which this is mentioned:
As you can see from the links, the authentication for an endpoint is configured when you set it up.
As for the alfresco integration, I may be wrong but I don't think there is anything that lets a user be chosen at run-time. I'm only seeing examples where that gets decided at design-time (unless it is the process initiator). The file and folder controls use the authentication set up when the repository was configured in APS. A publish or action step will be carried out as a particular user or as the process initiator. If as a particular user then that is stored in the process definition (in the xml).
Could you elaborate on your particular use-case that you're trying to work through? Are you, for example, trying to show a drop-down that contains a list of users from an alfresco content repository?