Custom DNS config

Hello Folks

I´m new to dreamhost and I am having trouble to set some rewrite rules that worked just fine in the last host. They just rewrite to

Dreamhost staff said the ServerAlias directive couldn´t be set for shared hosting plans and now I´m stuck having to configure one custom DNS to each merchant.

It will be a headache, but the problem now is I dont know how to fill the form on the custom dns screen. Type A | CNAME ? I thought it was A, then I get this error when I try to access the merchants subdomain url:

Site Temporarily Unavailable
We apologize for the inconvenience. Please contact the webmaster/ tech support immediately to have them rectify this.
error id: “bad_httpd_conf”

So I tried setting it to CNAME, but then it says ‘the value should end with a period’. I then put the period after the IP and it says the value isn´t valid etc.

What you guys advise me? Push them for serverAlias or learning this forever-time-consuming task?