How to Set Up and Connect a Secondary Email Domain in Google Workspace for Campaigns

Hi everyone, I’m struggling with something that should be pretty simple, and I’m sure some of you have already dealt with this. I’d like to use our secondary domains for Clay or Instantly campaigns, but I can’t seem to access or connect them properly, and I’m not sure what I’m missing. How do you usually set up and connect a secondary email domain? How do you set them up from you Google Workspace ? Do you have to create new inboxes for it, or is there another way? Thanks a lot in advance for your help!