If you have a facebook audience, then you need to use your custom domain/subdomain with your landing page.

Here below the reasons:

If you use zagomail link for your landing page which is like: https://zagomail.co/XXXXX

When you use the landing page link above , the facebook replace it to something like: https://zagomail.co/XXXXX/?fbclid=IwAR3PphUzF229KWDheb3FkuNC_4wE4RzEhegsuzFgxDTF9GVimYzVun6bo

This link will be broken because facebook pixel does not parse the XXXXX part correctly and the result will be a broken link.

For this reason If you have a facebook audience, then you need to use your custom domain/subdomain with your landing page.