How to log in an API created user to web UI?

We have a custom form which creates a user and then posts a comment to a specified board. 

We use /users/add to create the user based on information entered, and then /sessions/login to authenticate them to be able to continue with the script and post.

At this point we would like the user to be logged into the web UI of the community rather than just the api. Is there a way to achieve this without redirecting the user to the login page?