Hi ARanebo,
Before moving on, I’d like to confirm the following information with you:
1. Do you want to use the secondary domain as your SharePoint website address?
2. Are you using Office 365 for Small Businesses?
If both are yes, I’d like to suggest you change the website address to the secondary domain by following these steps:
1. Sign in to Microsoft Online Portal with your admin account.
2. Click “Domains” under “Management” in the left navigation panel.
3. Click “Change address” in the “Domains” page.
4. Select your secondary domain in the drop-down box and click “OK”.
Here is an article for your reference: Use a custom domain name for your SharePoint Online public-facing website address
Thanks,
Allen Zhang