You could try padded cycling shorts/pants, but that would add barely a centimetre.
Re-upholstering the saddle with a thicker cushion might help, but could be fiddly. Adding a block of wood securely between cushion and frame might work too.
Ultimately the most authorative answer will come from the manufacturer. I suggest you reach out to them and ask.
I suspect this is more of an electric motorbike with barely-functional "decorative" pedals, and just there to satisfy legal requirements. The footrest over the bottom bracket makes it seem like pedaling is not the intent.
If you get an answer, do please post back here to share for other future users who may have similar problems.
Another thought, if you slide yourself backward on the bench then that may give you more leg room, essentially riding it like a crank-forward bike. Downside here is the handlebars get further away which might not suit your posture.