The User-Defined Overrides section of an AP Group can be used to add configuration settings that are not already included in the UI. For Mss Value add the following to the User-Defined Overrides text box with the desired value:
MssValue=1260
A full guide for using the User-Defined Overrides can be found at the below Knowlege Base article. That article also goes into detail about how to set device-unique values using User-Defined Overrides.