Once you complete requirements the feature must be enabled by YouTube and THEN you must wait until YouTube actually offers you a choice of URL(s) to choose from:
If you’re eligible for a custom URL, you’ll see a notice in your basic info setting, get an email notification, and may see a notification in your Channel dashboard.
You get to accept or decline said offer. The timing of the offer is completely up to YouTube and there is nothing you can do to speed it up.
You can not create a Custom URL from scratch.