Make sure you're logged into the right account and follow these steps: https://goo.gl/ruFcXC . If eligible, you can claim a new URL right after, but it takes a few days for the old one to stop directing to your channel. Hope this helps.
To clarify, in the "Get a custom URL" box, you'll see the custom URL(s) you've been approved for. You can't change the part in the gray box, but you may need to add a few letters or numbers to make the URL unique to you.
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It looks like to you wouldn't be able to have a custom URL with just 'harv'. You'd need to add additional characters to create a unique URL for the channel. Let us know how it goes.
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