Launch the game and click on the Steam Workshop button inside the game.

I know this, but after ? Where can I submit my scenario ?
(I don't want download, I want upload !)

From Ozshara's modding guide: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=190070326

2. How to publish a mod to the workshop
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Ok, lets do the basics first. You have some files ready you want to test or upload.

2.1 Create a local mod
by browsing to your AoE game folder in your windows explorer. By default this will be 'C:\Steam\SteamApps\Common\AoE2HD'.

In there is a folder named 'mods'. This is the modding area that solves only one purpose - giving modders a place to create and test their creations.

In there, create a new folder named like you want you mod beeing named in the Steam workshop.
In your new mod folder recreate the folder structure from within your game folder needed to place all modded files at the same location inside the mod as they reside at in the game folder.

So in case you want to replace the soundtracks first track, take a look where the default file is placed.
It is 'C:\Steam\SteamApps\CommonAoE2HD\resources\_common\sound\music\xmusic1.mp3'

Therefore you need to create all those folders so you can place your modded version here:
'C:\Steam\SteamApps\Common\AoE2HD\mods\resources\_common\sound\music\xmusic1.mp3'

This is REALLY important, as a subscribed mod extracts all contained data to exactly that folder in your game directory, you chose/uploaded it from!

2.2 Add a thumbnail image and a description
by adding the corresponding files to your mods main folder.

The thumbnail is your mods main preview that will also be shown in the Steam workshop and really advertises (or doesn't) your mod as it will be the first impression people will have of your work.

For an image to be recognized as the thumbnail image it has to be named _preview-image.png!

If you don't add an image, in the process of uploading your mod, the default preview image will be added - but you don't really want that one, right?

While the need for adding an image here is pretty much self explanatory, the addition of a description isn't.

'I can do that in Steam itself' you might think - and you are right.
BUT in case you ever want to update your mod and there is no description present in your mods main folder on its first upload, the default description will be added to your mod like with the preview.

If there is a description present, it will be used though - this means your detailed mod description you might have saved no where outside Steam will be overwritten with the default one when updating your mod later - except you added your wanted description to the _mod-description.txt file.

This file is no plain txt file though, but needs to use a specific syntax. Easiest is to take a look at a downloaded mod for this. Just exchange that files text after the 'Value=' tag.

2.3 Test your mod now
by running the game.
You will see your local mod is automatically added to your load order that consists of all your subscriptions and all subfolders found in the 'mods' folder.

You can use it like any other mod now.

2.4 Uploading a mod
works the same way.
When selecting an local mod, you will see a 'publish' button beneath the mod list.

Left of it you can also check all legit content of this mod (this is somewhat irritating - EVERY content your mod might contain will get published - but only the files replacing default ones are listed and used for the estimated mod size).

In the process of publishing a mod, you get a second entry for this mod in your mod list.
This is for you are subscribed to your upload too.

So now you have the uploaded and local version as separate entries.
This is pretty helpfull as you can at every time compare your local copy to the uploaded one this way when applying changes later on.

2.5 Updating a mod
works exactly like publishing it.
Select the local version of your mod.

In case you have no subscription of the uploaded version at this point, your local copy can not be linked with it and therefore counts as a new, never before published mod.

Publishing it means that there are two uploaded mods then.

When you are subscribed to the uploaded version as well, the 'publish' button is now labeled 'update'. Clicking it will replace the uploaded version with your local one.

An important note - the local and upladed mod versions are linked through their name.

So in case you edit your mods title in steam, you NEED to also rename your mods local folder. Otherwise the local mod will not be detected as your authors copy of the uploaded version even if you are subscribed to it.

Thanks !