I don't know if there's a way to do this through the builder (I doubt it), but I believe you can kind of hack it by changing line 971 in src/app/storymaps/common/Core.js
from...
forceMobile = _urlParams.forceMobile !== undefined || app.indexCfg.forceMobile;
to...
forceMobile = true;
But if you're not building the web app from source, it's a pretty long way around to add this functionality.