All our paid servers come with the ability to use custom jars by default. To use a custom jar, follow the steps below:

Step 1: Upload the jar file via FTP to the root directory by going to Files -> FTP File Access -> Login -> Upload. If you are uploading a specific Forge version then it's best to zip up the two jars along with the libraries folder and use the 'Archives to upload and unpack' option on the 'upload' tab.

Step 2: Go back to your server page on Multicraft and select the 'Custom Modpack/Jar' jar dropdown option and then rename the custom jar in your FTP to custom.jar.

Step 3: Start the server and you're all set!