Branded URL is only available from the Pro plan. See pricing here.
If you're on a trial or the Pro plan, visit the settings tab in your PixelMe's account. Then, redirect your shortlink DNS to our server using these two solutions: Create a CNAME record pointing the desired domain to cname.pxlme.me OR Create an A RECORD record the desired domain to 18.104.22.168.
Be careful, in order to fully work, the redirect can take up to 24 to 48h. When the redirect is pointing to pixelme.me, add you branded domain in Settings tab under 'Custom domain'.