Hiding Sub-domain URL address in cPanel?

Hi Everyone, I'm very new to this, so I would appreciate any help. My main domain is /http://www.mariasmusicschool.co.uk/ Today I purchased a new url /http://www.arminiancaptivity.co.uk and I added this to my account in cpanel and redirected the new url to /http://www.mariasmusicschool.co.uk/arminiancaptivity which is working perfectly.

Now, here is my question. When I type /http://www.arminiancaptivity.co.uk, how do I hide or mask the url address, so that it doesn't appear in the browswer as '/http://www.mariasmusicschool.co.uk/arminiancaptivity'? In other words, how do I hide the primary domain url, so that it appears as if it's in it's own url /http://www.arminiancaptivity.co.uk

I hope this is clear, as I'm very new to all of this. Thanks so much in advance! Maria.

 

Bump, Bump, Bump... Can anyone help me? I want to hide the primary domain address and only have the second domain appear if you understand what i mean. Thanks in advance. Maria

 

Ughhhhhhhhhhhhh!!!!! I didn't realize I was redirecting to .htm and because my file was .html, I was getting no where. Thank you for the help. I had already used the Add-On-Domain, but your assurance made me check everything again, and I found my stupid mistake. Thank you!!!!!!!