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.
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.