Hi Popejmp,
To verify your domain in FB Business manager you need to follow this Verifying Your Domain. Once you complete the guide. locate your Meta Tag Verification Code in FB Business Manager business settings> brand safety> Meta Tag. The code will look like this:
Copy that code.
Login to Shopify
Online store > Themes > Edit Code > Theme.liquid
Locate the ending header tag in the theme.liquid file and paste the code right above.
Go back to FB Business Manager, business settings> brand safety> Meta Tag > Verify