If you'd like to install add-ons to your server's world then you're going to want to start off by downloading your world from the server.
Step 1: Downloading Your World
Head over to the Game Panel and stop your server.
Navigate to the "Files" tab and open the "worlds" folder.
Select your world and archive it by right-clicking the folder.
Now download the "archive-.....tar.gz" to your computer by right-clicking it.
Make sure to extract the archived file after the download is finished.
Now that you've extracted the world, put it to your Minecraft worlds folder.
You can find the worlds folder by pressing Win + R and then pasting the following path into the run window that opened:
%localappdata%/Packages/Microsoft.MinecraftUWP_8wekyb3d8bbwe/LocalState/games/com.mojang/minecraftWorlds
Step 2: Installing the Resource Packs/Addons
After you've placed your world into this folder then you should be able to see the world in your worlds list.
Now just click on the edit button next to your freshly added world.
Navigate to Resource Packs or Behaviour Packs tab from the left and click on "My Packs".
Activate your desired add-ons.
With some add-ons you'll also have to enable some or all features under 'Experiments'
Step 3: Uploading The World To The Server
Now just upload your world with activated add-ons back to your server. Rename one of your world folders so the worlds wouldn't be with the same name.
Done! Now you can navigate to the "Worlds" tab and activate the world that has the added resourcepacks/addons.
🎉 Congratulations, you've successfully added resourcepacks/addons to your world. If you have any issues or concerns, please contact us through the ticket system.