First, download the installer version of FabricMC from here.
Select the installer version

Double click the downloaded file to open it.

Select Server then select a folder for the FabricMC files to be installed to.
Installing Fabric

Navigate to the folder you just installed FabricMC to, inside of it you should see two jar files and a folder. Upload these via FTP or the File Manager.
Uploading the fabric files

In the game panel, change the jar to custom.jar in the dropdown box then paste the name of the FabricMC jar you uploaded via FTP. This should end in -launch, not -installer.
Changing the JAR in the panel

Finally, start your server. You should see a mods folder was generated, which you can use to install mods.
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