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!
Mr.Aces's post in Discord Widget was marked as the answer