Requirements:

A FreeMC.Host Minecraft Server. Create one here for free

The .jar file of the custom version that you want to use

Step 1: Open the website

Step 2: Click Advanced Settings on the server

Step 3: Select "Custom server.jar"

Step 4: Open the panel

Step 5: Click on the server

Step 6: Open File Management on the left

Step 7: Drag the .jar file to the middle

Step 8: Rename the .jar file to "server.jar"

Step 9: Restart your server