This will only work if you have administrator access.
You first need to have an exclusive IP address set for your domain. If you don't know how to do this, go to the client's account and click the IP Pool icon. Add the corresponding IP address there.
Change the physical hosting setup for your domain to use the IP. (Client > domain.com > Setup).
From there, go to the server tab and click IP Addresses. Click the Hosting column of the corresponding IP address. Click the radio box and then Set as default.
Should be working.
HTH,
Steve