The below would set the Profile URL field automatically using CB Auto Actions.
Global
Triggers: onAfterUserRegistration, onAfterNewUser, onAfterUpdateUser, onAfterUserUpdate
Type: Field
User: Automatic
Access: Everybody
Action
Field: Profile URL
Operator: Set
Value: [username]
This would set it to username. CB Auto Actions has built in Profile URL field support so it will safely convert it to a URL safe alias and check for duplicates (it will skip updating the field if a duplicate is found, but should never happen since usernames are always unique).