With Boei, you can create a Discord button widget for your website within minutes. It is super simple!
How to get there: Go to Setup → Widget in the top menu → click your widget → click Add Channel and select Discord.
See Installation Guides.
Open Discord, click the gear-icon next to the channel you want to use and go to Invites.

Create a new invite and ensure that you click the checkbox next to "Set this link to never expire".

You now get an Invite URL from Discord. Copy-paste the part after discord.gg/ to your Boei channel setup.
Give it a test to see if everything is set up correctly.
Done!