Got it, thanks a lot! 😄
I want to quickly document the process because it wasn't clear to me.
1. Make your mod. Test it in your game's data folder (the one you had to make manually).
2. Make a new folder called data somewhere on your hard drive (such as C:\Homecoming_Mods (don't have spaces in the name)).
3. Put your mod in the correct folder structure inside the data folder. (e.g. C:\Homecoming_Mods\data\sound\ogg\powers).
4. Open the Mod Manager and go to Tools -> Create mod.
5. Put a name in the box and click the Browse button - find the data folder you just created.
6. Click BUILD. This will put a PIGG file in the folder which contains your data folder (in our example that's your C:\Homecoming_Mods folder).
*see below
7. Log into the website: https://mods.cityofheroes.dev/
8. Click Upload Mod.
9. Fill in the fields.
10. Click Choose Files, select the PIGG file you just created.
11. SUBMIT!
12. You probably want to delete the data folder you created or it's contents or move it somewhere else before working on another mod.
That's it! Thank Michiyo for their hard work!
11.5 If the website produces an error when you click Submit, navigate to your mod again, click [edit], Choose Files again and Submit again. This was wrong.
*6.1 It might not be a bad idea to remove the 'test' version from one's <CoH install>\data folder (as mentioned in step 1), and
6.2 Copy the newly-created PIGG file to one's <CoH install>\assets\mods folder
6.3 Verify the mod works in-game before uploading it to the City Modder website.
Thanks to @AboveTheChemist