Right now there is no way to trigger the mobile menu completely on demand, but weaver added the threshold option box that you can use to make the menu switch to mobile at a wider smaller width than 768 which is the default threshold. (edit by weaver)
The option is in the Menu options towards the bottom.
The other solutions is to use CSS to make menu labels fit using font size, padding. There may even be a way to make labels changes to shorter ones using before or after selectors, I can explore that if you need.