Is it within Facebooks App Policies to have a form for an action in my app that includes a pre-checked checkbox to share the story on Facebook (which then creates a post after submitting the form)?
I can only find this relevant part in their policies (not guidelines):
If a user grants you a publishing permission, you must still obtain consent from the user before taking any action on the user's behalf, such as publishing content or creating an event.
And this part in the Platform Policy Checklist:
App does not automatically post Stream stories on a user’s behalf and instead obtains consent by providing users with an option to click a button or check a box that clearly explains their content will be shared.