First, check out the general eligibility requirements here: https://yt.be/help/4moa . If you're eligible, you should see a notice in your advanced account settings. And to get set up, just follow the steps outlined in that same help resource!
Update – The custom URL you're trying to get is unavailable for claiming. You can choose a different one or add a suffix instead on your previous choice. Follow the steps to set up a custom URL: https://yt.be/help/4moa & reach back if you need more help!