To download the modpack server file to your server, you will need to go in to 'File Management'. The Download to URL way (Requires a DedicatedMC Server).
For this tutorial, we will be using Sevtech Ages. If you want to install a CurseForge modpack on your server, there is 2 ways to do it. For this tutorial, we will be using Sevtech Ages as the example. This tutorial is going to show you how to install a CurseForge modpack of your choice on a server.