Using WSAPI use this query:
https://rally1.rallydev.com/slm/webservice/v2.0/user?query=( username = <User>@<Company.com> )&fetch=objectid
The above query is requesting the 'objectid' for a specific user. Notice the 'fetch' argument that's requesting the 'objectid'. The result is:
If you omit the Query argument then you it will query the requester's own user object. This example, retrieves the objectid for the logged on user , the one who is making the request:
The above demonstrates how to query a user (or if no user specified then your own user). The 'objectid' can be replaced or extended with any other fields of the user object that you may need to fetch. For example using:
fetch=objectid,username,emailaddress will return objectid , username and email for the user that's queried.