Whether you need to verify your domain so you can use the Facebook Pixel for your Ads, or add some meta info to your project, you can use Meta Name and Meta Content.
To access the Meta tag settings, navigate to the settings of your project and click on "Manage Domains"
On the new screen, Select "Meta Tags & Verification"
Enter the Meta Name and Content you want, you can add up to 10 meta tags.
Facebook Pixel Domain validation
To specifically address the Facebook Pixel verification requirement (Facebook Business Ads feature), use the Meta Name: "facebook-domain-verification" and the content chain you will get from your Facebook Business on the Meta-tag Verification tab:
Additionally, Facebook is checking the exact path of your subdomain. Make sure you have a start project in the domain settings.
For more instructions about Facebook Domain validation, please check:
If you have any question, please contact support.