by Nik Okuntseff.
A user account for your web service is a normal account in Moodle, so you create it in a similar way. Manually - in Home - Site administration - Users - Accounts - Browse list of users. Click the Add a new user link.
But the additional steps are to create a new system role, assign capabilities to the role, then assign user account to the tole. Take a look at the link in my previous post, it explains the procedure with screenshots.
Capabilities needed depend on the function your web service uses.