Step 1: Log into your Facebook Business Manager account at https://business.facebook.com/
Step 2: Navigate to your client's account > Brand Safety > Domains
Step 3: Click "Add A Domain" and enter the url without the https://
Step 4: Copy the text record shown in the DNS Verification tab
Step 5: Log into the domain's DNS manager and create a new TXT record using @ for the host name and paste the value you copied from Facebook Business Manager. Save the new record.
Step 6: Go back to Facebook Business Manager and click the Verify button.
When you verify the root domain, all associated subdomains are automatically verified as well. To do this, navigate to your Pixel settings and locate the "Domains on your allow list" section. Add your root domain, save the changes, and you'll notice a confirmation stating, "your domain and sub-domains."
When setting up your Facebook ad, enter the sub-domain in the "Website URL" field, and you'll see an option in the Tracking section to choose the related root domain.