You can write your topic however you want, but you need to answer these questions:
The Roblox Policy service AllowedExternalLinkReferences allows us to decide what players are allowed to see with regards to social media.
To that extent, are we allowed to use “Join our Discord” as a private message to the player’s chat who has this enabled.
Is it within the rules?
Yes, you can do this along as AllowedExternalLinkReferences allows you to show Discord.
Some games ask you to join their discord, follow them on twitter, instagram, youtube, tiktok, all in one billboard. So I think just discord would be fine for your game. You’d have to use that policy service, I’d imagine. Never heard a story of roblox getting mad at someone for not using it, though.