Stripe one-off sign up fee with monthly recurring subscription

Hi all,
Im having trouble finding how to add a one-off sign up fee for my app.
That is: the app costs $16 per month but users need to pay a $10 sign up fee.

Online it says the way to do that is with an invoice but I cant see how to automate that. I tried to add the $10 as a seperate product but the bubble workflow options only allows for subscriptions as far as I can tell.

Any help would be much appreciated