First, you have to start your match by going to Multiplayer -> Network. This is important! You will need to attach a replay file to your score, and since the game doesn't create replays in the singleplayer, you have to use this method.
Don't forget to take a screenshot at the end and save your replay! The last Replay should also always be saved in your replay folder under the name LastReplay, in case you forget saving in the heat of the moment. You can find the replay folder here: C:\Users\<your username>\AppData\Roaming\My The Lord of the Rings, The Rise of the Witch-king Files\Replays.
The next step is to upload the screenshot, for example on one of the many image upload pages on the internet like
imgur.com or on Discord.