Your Sage HR domain is the bit before sage.hr in your company's Sage HR URL. Your domain ensures you log into that specific company on Sage HR.
How do I find my company's domain?
From your welcome email
When an employee receives their welcome email or any Sage HR notification email, they have a link to your company's page. The domain is in that web URL.
If you click on the link this takes you to your company's Sage HR login page. You should bookmark or favourite this page for quick access going forward.
Look at company's Sage HR URL when you're logged in
Just check your URL bar when logged into your company.
Ask your employer
Your employer or HR admin can inform you what the company's Sage HR domain is. If they're unsure they can find this out from Sage HR after passing security questions.