You can verify your domain's ownership in two ways:
Add a meta tag to your website's code
Within the builder, access the website's integrations settings, find the Custom code field, and insert the given verification code there:
Save the changes and update your website. Then, verify your domain on Facebook's end.
NOTE: In the Custom code field, you can paste as many codes as you need, simply paste one below another (don't paste a code in the middle of another code)