Once you have finished creating your scenario, there are several options for sharing it with others.
To share the scenario online directly, expand Scenario option in the menu bar and pick the Publish to Steam Workshop option. A window with upload settings will appear. Make sure to add either Singleplayer or Multiplayer tag, it is important for filtering.
After agreeing with the Workshop license, you are free to publish the scenario and share the link with others.
You can also export the scenario into packed *.pbo format. Expand Scenario > Export and pick either Export to Singleplayer or Export to Multiplayer. The scenario with all files and folders inside will be packed into a *.pbo file. You can find it in the game's root folder in the folders named Missions and MPMissions respectively.
If anyone else places the file in the same folders, they will be able to play the scenario from the main menu.