Years ago I created a 300 scenario with the battle of the thermopiles, for Age Of Epires II HD.
and now I want to publish it for Age Of Empires II DE, but I have not found the way, since I do not see a “workshop”.
How and where can I go about publishing the scenario I create? 
You first need to find the HD version of the scenario. The file path should be C:\Program Files (x86)\Steam\SteamApps\common\Age2HD\resources_common\scenario, but I haven’t confirmed that currently, although it gives you the idea. Find your file, copy it, then open up AoE2 DE, find the file location, and copy your scenario into it. Then to share it, go into the mod manager, open one of your mod directories, go out until you can see a folder called local, enter that and create a new file with the name of your scenario. Under that, create a file called resources, then some other files inside that. I think it will be common and then scenario under that, but I would need to check. Inside the final folder post your scenario again. Then go back to the ingame mod manager and click download local mods. Then share it. Hope that helps, and I might update this later, probably with pictures and stuff. Your scenario will likely be broken by DE though, just a heads up.