Email Campaign Header Linking

My company owns a school and a testing facility, both under different business names and URL's.

Squarespace hosts one of the companies, but not the other. I would like to use Squarespace as our email campaign provider both companies; however, when I create a newsletter for the company that isn't hosted by SS, the header of the newsletter only links to the company they host. We can't update the email header link anywhere.

Is there any custom code we could use to work around this problem? I'd love to use Squarespace for emails for both companies, it's so much better than the alternatives, but if we can't figure out this issue, we'll need to go elsewhere for email for both. Thanks for the help.