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, choose your desired domain and select "Meta Tags & Verification" from the dropdown menu.
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.
Facebook also advises to connect the verified domain to a Facebook page (under Assets): https://developers.facebook.com/docs/sharing/domain-verification/verifying-your-domain (this information is towards the end of the article).
This should at least allow for editing of the Facebook post elements (one can then change the short text attached to the URL of the landing page).
Once you're done setting everything up in involve.me, make sure to return to the domain verification settings in Facebook and click on "Verify".
It can take up to 72 hours until Facebook finds the meta tag code.
You can confirm if the meta tag is listed by going to the scrape results in the Sharing Debugger Tool: https://developers.facebook.com/tools/debug/
If you still have any questions, please feel free to contact support.