I am not sure I am completely understanding you yet, but you will find instructions on installing the Protomail bridge and configuring Mail here. Once you have done that and paid to use Protonmail's servers you can configure Mail to prefer Protonmail for sending messages and invitations to appointments scheduled in Calendar will be sent via that link.
"All you've got to do is own up to your ignorance honestly, and you'll find people who are eager to fill your head with information" --Walt Disney
