You know how you post a mod's numerical code into the launch properties in order to make the game load it? Well, to add a SUBMOD, you simply add the submods code after the main mods code.

This submod page has some clearly labeled instructions for how to do it, though they would apply to any other submod.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3489324011