If you are using secondary domains, it is recommended to set up forwarding to your main domain. This way, when the clients enter your sending domain into a browser's address bar, they will be taken to your main website.
Watch the detailed video by clicking the link here.
Here are the steps to set up domain forwarding in Namecheap:
Log in to your Namecheap account.
Navigate to Domain List, find your domain, and click Manage.
In the Domains tab, scroll down to the Redirect section.
Click Add Redirect.
In Source URL, type your secondary domain, and in Destination URL, type your primary domain. Remember to include http:// or https:// with your primary domain.
Click on the check mark to save changes.
Now you are all set. When clients who receive your email click on your secondary domain, they will be redirected to your main website.