Hi,
Below is the problem we are trying to resolve. 
- We have bought an Appointment booking product (Booking Bug)  which will be added to our customer community.
- Community user will click on a button/link to book appointments 
- Appointment booking screen opens in an iFrame
- user books the appointment
- At the end of process we want the appointment system to auto populate logged in user details ( Contact Name, email, Account name, Number)  on the appointment  booking form.

we are exploring the use of connected Apps to get the details however there are challanges with that. 
Are there any mechanisms/solutions to resolve this problem.

Thanks,