You may not be able to do what you need to do from the DirectAdmin control panel. If you have your own server administrator you should have him or her take care of it for you.
Otherwise, from the admin login (not the account login) of your DirectAdmin control panel check if you can do a custom HTTP configuration to do what you want.
If not, then you'll need to create a custom domain-level httpd.conf change from a root shell login. There have been many posts asking and answering how to do this, already on these forums. I'd recommend either searching for them, or hiring me or someone else to handle your server administration.
I'm not sure what you mean by a co-hosted server. My company, and other companies you'll find posting on these forums, offer completely managed Dedicated and Virtual Private servers running DirectAdmin, and if you don't know how to manage your sever, having a managed server may be your best option.
I can't give you complete instructions on how to do it, because I don't know the changes which need to be made. Even if I did, I would be loathe to just give you a recipe for making the change; with one simple typo you could cause major problems on your server.
Jeff
