Note: Before mounting a phone on your motorcycle consider the following:

• Phones are hard to read in strong direct sunlight
• Phone can overheat can overheat easily and switch off
• Phones are difficult to manipulate with gloves
• Phone with moving parts (like optical image stabilization for example in your iPhone or Galaxy) are known to break when subjected to the vibrations of the motorcycle.

KTM offers a bracket for handlebar mounting p/n 61712991100 at £61, then the case for the phone into which the bracket clips is about £35 (different part numbers for different phones).

Plenty of third party provide phone mounts, and using a GPS mount with some RAM hardware is probably the better option as it moves the phone higher in the field of vision. A quadlock on a RAM ball is quite practical for example. Or a X-Grip which also mount with a RAM ball.