The attached file will verify that the custom user field has a valid, paid, Minecraft user. Download and extract the Minecraft.zip attached below. Upload the Minecraft folder to your /library/ folder, creating /library/Minecraft/UserField.php Create a custom user field as a single-line text box specifying whichever options you want at admin.php?user-fields. The only requirement is that the PHP callback in the "Options for Text Fields" is set to Minecraft_UserField::verifyAccountName You also need to create a new XenForo phrase named no_paid_minecraft_account. It should now work.