To create a ModMail thread, you will need to first DM MailBox. To trigger the opening process, simply type any message. After that, MailBox will respond with a list of mutual servers. From this list, select the server you wish to contact and run -open [id] with "id" being the associated ID.
If all goes well, a channel will be created in the selected server, allowing a two-way communication channel. Users who have access to that channel and have a role added through addrole can now reply to any message you send in DMs.
Sending messages to the specified server is extremely easy. As long as you have a thread open, just DM MailBox your message and it will be redirected to the server. Its as simple as that!
If you get a success message, then the task has been completed successfully! Your message has been relayed to the server, and they can handle the rest! Check out the next guide, "Managing Threads" for information on how to reply, manage, and close threads.