How to install mods in Killing Floor 2

First, please stop your server.
Once your server is stopped, switch it to Advanced mode.
This is necessary because we need to edit the PCServer-KFEngine.ini file.
Click on PCServer-KFEngine.ini in the white column on the left.
In the resulting text editor, add the following line anywhere:
[OnlineSubsystemSteamworks.KFWorkshopSteamworks]

Next, you will need the WorkshopID of the mods you want to use.
You can find this in the URL in your browser: https://steamcommunity.com/sharedfiles/filedetails/?id=2222630586
In this example, the ID would be 2222630586.
Add it below the line you created earlier, like this:
ServerSubscribedWorkshopItems=2222630586

The final result should look like this:

    [OnlineSubsystemSteamworks.KFWorkshopSteamworks]
    ServerSubscribedWorkshopItems=2222630586

For each mod you want to use, you will need to add a new line.
With multiple mods, it would look like this:

    [OnlineSubsystemSteamworks.KFWorkshopSteamworks]
    ServerSubscribedWorkshopItems=2222630586
    ServerSubscribedWorkshopItems=643383080
    ServerSubscribedWorkshopItems=682290186

Next, find the section [IpDrv.TcpNetDriver].
Add the following line below it:
DownloadManagers=OnlineSubsystemSteamworks.SteamWorkshopDownload

Finally, save your changes by clicking Save.