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 txt 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.
Verifying the root domain automatically verifies any sub.domains of that root. In your Pixel settings, scroll down to the "Domains on your allow list" section and add your root domain. Once you save, you will see it says "yourdomain and sub-domains"
Then when you are building your Facebook ad and you enter the sub-domain the "Website url" field, it will give you the option to select the corresponding root domain in the Tracking section.
The team at sol8.com confirms that verifying the root covers the sub-domains