Go into your AdminCP and then in your pages/block section.
Add a new block with name "Discord Server". Add template key i.e discord-server
Now go in content tab and paste this code:
<a href="Your_Server_Invite_Link_Here"><img src="https://discordapp.com/api/guilds/Your_Server_ID_Here/widget.png?style=banner2" alt="Official Discord" style="max-width:300px">
Replace server invite link and your server ID. (Hint: You can get your server ID in your discord server settings)
You can also change the banner style i.e banner2, banner3 etc
Now add the block on your forum homepage!