Your service provider suggests the following:
Free accounts use the eWedding URL you chose at signup while upgraded
Premium accounts may have chosen a custom .com. Even if you are using
a custom domain name, your original eWedding URL will still work.
However there is a way as you have described to purchase a domain name
example.com and set up a webpage containing a frame redirect to
https://app.ewedding.com/example.com. In order to do that you 'll need to create a webpage at
example.com/index.html with this code here:
<html><head><title>Title of your webpage</title></head>
<frame name="main" src="https://app.ewedding.com/example.com " scrolling="auto" noresize>
Your browser does not support frames
This method is called Frame Forwarding or Cloaked / Masked / Stealth Web Redirection. Many domain name registrars provide this option without necessity of obtaining an actual hosting and having the above coding done.
You should also carefully read terms and conditions of this ewedding app service in case such a way of using it is not legally allowed.