To publish a page on a custom domain, go to the projects screen first and find your page there. Click on "Actions" and then on "Publish"
If your campaign is not yet published, publish it first by clicking on the "Publish project" button.
After you have published your page you can click on the Add domain button:
Here you can enter your root domain e.g. Google.com. Enter yours and click on next:
Now enter your subdomain e.g. landingpage --> This means your actual domain name will be landingpage.google.com if you use google.com as your domain name.
After clicking next you can follow the instructions: (below screenshot is an example, please check your own settings)
After you've done all the steps it can take up to 24 hours before the domain is propagated:
You can also check your dns by clicking on 'test DNS'' :
