Editing unlinked page domain

I am essentially wanting to create a landing page with a custom domain or at least a custom domain linked to my website. I am essentially wanting to use it as a sub-url for this ad campaign
is there a way to do this that I am missing? 

customname.com -> landing page // website.com/ad

Hi Allie,

So when people click on an ad what domain do you want them to be directed to? 

Are you wanting them to go to landing page on the square space domain and then be directed to your actual website? 

Cheers Andrew.