First off, make sure to operate on a operating system higher than OSX Yosemite.
Connect your iPhone or iPad via cable to your Mac, allow all connections on the device and the computer. Then, open Quicktime.
In Quicktime, go to File -> New Movie recording and click.
A new window will open up with a view of your webcam.
Here, you can choose the source of the video through a small dropdown menu right to the record button.
Select your device and it will be displayed on your screen.
In Demodesk, open a new tab in a meeting and select "Screen" when asked what you want to share.