If you've already added hosting for the site, it's under Domains>Manage Domains. In the table where you see all your domains and subdomains, there's a column for "Web Hosting". In this column is a button named "Edit". (note that there are "Edit" buttons in other columns, you want the one in the "Web Hosting column)
Now you're at the screen where you're editing the hosting for your domain or subdomain. Towards the top it should say: "Now editing the hosting for mydomain.tld:"
On this page there are several sections. The way it works is that you can choose how to host the domain and then click on the blue button in the section. You want the second section called "Redirected". The button for the section is called "Redirect this domain now!"
Hope this helps!
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