It's really easy to do! You can watch this video tutorial to see how (It might be for a different system, but all the techniques will be the same):

Or you can follow these instructions:

1. Login to your domain account. 

2. Locate your domain in your account that you'd like to use with your marketing system. 

3. Locate where you can edit the DNS Records or Set Nameservers for that domain. 

4. On this page, there should be an option where you can set a URL to forward your custom domain to. Make sure when setting up this section, that you choose Forward Only. If you try to do Forward with Masking, it will cause an error when forwarding to the marketing system URL. 

5. Once you have input your main website URL in this section, save it. Then you're good to go!