There are essentially two ways how you can make your website that was made in Website Maker work together with an externally hosted domain name:
1.) Some domain hosting packages will give you the possibility to set up a simple URL redirection. You will have to log into their administration area and look for the forwarding/redirection feature. You will have to specify a target URL, which should be the address of your actual Magix website (the sub domain name). Done. If someone visits your external domain name now, they will be taken to the Magix website. If available as an option, make sure to activate URL masking, so that you domain name will be displayed in the browser's address bar the entire time.
2.) If a simple URL redirection feature is not available and there's some webspace available as part of your hosted domain, you would host a small piece of HTML code that basically just displays your Magix site within the other website. You can obtain the required HTML code from the "Export" function within Website Maker ("More" menu).
So, these are two possible solutions, with the first one probably being the more simple and straightforward one, if supported by your domain hosting.