Hi Leanne,
You will need 4 x M8 screws, 20-22 mm length, and 1.25mm pitch for the TU690T. The 20-22mm listed in the user manual is not the screw length. It’s the wall mount thread depth in the TV chassis.
Make sure to add the wall mount bracket thickness to the thread depth for minimum screw length.
If you cannot find the fitting screws, you could use washers (supplied with the wall mount) to adjust the length.
If there is anything we can do to assist further and improve your user experience, please let us know by contacting Samsung Customer Support at 1-800-SAMSUNG, 8 AM - 12 AM EST, seven days a week.
~Samsung Advisor