When you're ready to publish your mod, go to common\XCOM 2 SDK\XComGame\Mods\YourMod and open YourMod.XComMod in a text editor. Add "tags=" to the last line (or above "contentImage=" if that's there). Tags are comma separated and lower case, only the default tags visible on the workshop will actually function for filtering purposes, AFAIK you can add non-default tags but it's no different from just being descriptive in your mod's description. Here's my tags line for example from line 5 of my HavocVoicePack.XComMod file. The spacing is important.
tags= voice, soldier customizationAfter adding them there they should show up on the publishing menu, when you update your mod you'll have to re-add the tags from what I remember reading (just like you have to re-update your description on Steam)
