I would never recommend sharing your account credentials with anyone. Only trusted parties should have access, and even then Cloudflare has the facility to invite users to an account without having to send your credentials.
This seems to be the post they are referring to → Need help on SSL activation
Reading through their docs, the way they want you to setup SSL at Cloudflare is not secure and simply hides the fact that they do not have a certificate for your domain. If they want to offer custom domains with HTTPS, they will need to provide that. Using Cloudflare as they describe it would encrypt the connection from your users to Cloudflare, but the certificate between Cloudflare and Pabbly cannot be verified as it is not valid for your site. This is an insecure setup, and is especially bad for a provider that appears to accept payments.