User ID field is not an auto-generated numeric key in our system, it is a required "string" type in EasyLink API that you must enter, but in order to keep backward compatibility we use it as an eight (8) digits number, so you need to make sure to use the correct format, this allow you to use your own User ID to register users into our system if you use numerics IDs, that way you can have everything sync without to much troubles, however we have the Personal Identification field for the users, which can be any string and you can filter users and time logs using this field too, also we have 2 custom fields for externals IDs (ExtId1 and ExtId2) just in case you need some extra fields to put some IDs for users.
This are invalid user IDs
"MST-111023". A Correct UserID should be any number
from 1 to 99999999.