You will need to install forge to use mods on your server, if you're looking to install a modpack there is a video tutorial on this.
First, download the installer version of Forge from https://files.minecraftforge.net/, making sure to select the Minecraft version you wish to use.
Double click the downloaded file to open it
Select Install Server then select a folder for the forge files to be installed to
Navigate to the folder you just installed forge to, inside of it you should see a folder named libraries, and two jar files. Upload these via FTP.
In the Multicraft panel, change the jar to custom.jar in the dropdown box then paste the name of the forge jar you uploaded via FTP. This should end in -universal, not -installer.
Finally, start your server. You should see a mods folder was generated, which you can use to install forge mods.
Video
First, download the installer version of Forge from https://files.minecraftforge.net/, making sure to select the Minecraft version you wish to use.
Double click the downloaded file to open it
Select Install Server then select a folder for the forge files to be installed to
Navigate to the folder you just installed forge to, inside of it you should see a folder named libraries, and two jar files. Upload these via FTP.
In the Multicraft panel, change the jar to custom.jar in the dropdown box then paste the name of the forge jar you uploaded via FTP. This should end in -universal, not -installer.
Finally, start your server. You should see a mods folder was generated, which you can use to install forge mods.
Video
Updated on: 26 / 11 / 2020