To get a custom URL for your channel, your account needs to be in good standing and meet a few other requirements:
500 or more subscribers
Channels is at least 30 days old
Channel has uploaded a photo for the channel icon
Channel has uploaded channel art
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Get a Custom URL for your channel
Once you’ve qualified for a custom URL, you’ll receive an email notification and a notification on your Creator Studio dashboard. To get your custom URL:
Sign in to YouTube and navigate to the Creator Studio.
Look for your “Get a new custom URL” notification and click in the included link
You’ll be redirected to claim a custom URL.
You’ll see the custom URL(s) you’ve been approved for. These cannot be changed. You may also need to add a few letters or numbers to make the URL unique to you.
Click the box next to “I agree to the Terms of Service” then click Change URL in the bottom left corner.
This URL will be linked to both your YouTube channel and your Google+ identity. Once your URL has been approved, you cannot request to change it. When you're certain, click Confirm choice.