Hey @abasille 
The user_id you specified is actually the correct one.
Since the POST /dbconnections/signup endpoint only works on database connections, the user_id will always be of the format: auth0|{_id} where {_id} is simply the value returned by the call the /dbconnections/signup. In your example: _id = 5aab90daaa9ad130c17483f8.
You can simply append auth0| to the _id returned and you should be good to go 