Are you experiencing any problems on your end? Could you please follow the instructions below:
you can do that only if you have a "Freelancer Plus" membership. It will cost you $10 per month. You can do it by following the steps:
Go to "Settings"
Select "Membership & connects"
Click on "View or edit membership plan"
Click on "Sign up" for Plus
And follow the instructions to pay 10 bucks.
But honestly saying, this is not necessary. Creating custom URL will not going to help some extra. But the Plus membership plan will give you extra 10 connects and rollover connects.
Hope you have your answer.