Requirements:

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

Step 1: Open the panel

This is located at panel.freemc.host - you should be logged in automatically with your account

Step 2: Click on the server

These are shown on the homepage of the panel in a list

Step 3: Select File Management on the left

Underneath the console

Step 4: Click on server.properties

If it's empty, start your server and it should automatically generate

Step 5: Find online-mode=true and change it to online-mode=false

This is the key step

Step 6: Click save and restart your server

Open your minecraft client and check if it works