Whether you need to verify your domain so you can use the Meta Pixel for your Ads, or add some meta info to your funnel, you can use Meta Name and Meta Content.
To access the Meta tag settings, navigate to the settings of your funnel 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.
Meta Pixel Domain Validation
To specifically address the Meta Pixel verification requirement (Meta Business Ads feature), use the Meta Name: "facebook-domain-verification" and the content chain you will get from your Meta Business on the Meta-tag Verification tab:
Additionally, Meta is checking the exact path of your subdomain. Make sure you have a Start Funnel in the Domain Settings.
Meta also advises to connect the verified domain to a Meta 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 Meta post elements (one can then change the short text attached to the URL of the landing page).
Important
Once you're done setting everything up in involve.me, make sure to return to the domain verification settings in Meta and click on "Verify".
It can take up to 72 hours until Meta 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.