If you are using a secondary domains for creating sending account, it is recommended to set up forwarding to your main domain.
If you want to set up the forwarding in Google, kindly follow these steps.
Log in to your Domains Page on your Google Domains.
Click Manage and select Website on the left side bar menu.
Click Forward Domain.
Enter the name of primary site in Website field.
Click on Advanced Options.
Select Permanent Redirect (301) for Redirect type.
For Path Forwarding, select Do not forward.
In Forwarding over SSL, select SSL On.
Once finished, confirm it by clicking Forward.
Learn more about scaling email outreach with secondary domains.