if using scriptsAre you using PlayMaker NGUI scripts -->
Asset Storetrue If so, then it's rather easy, just select the option shown in the screenshot I'm attaching.
In the screenshot I hoover over then even you are looking/asking for.
elseHowever If you're not using the script addon, you would have 2 options
either write an action yourself or find one.
Or access the function via scripting like NGUI was originally designed for.
NGUI add's the scripts automatically when you assign a sprite as a button,
you just need to perform the checks for when the user is accessing the GUI.
end if