You can't catch that directly through the menu system. Newmenus are supposed to hide pagination and will handle those events internally. The menu callback will not be executed:
https://github.com/alliedmodders/amx...a_api.cpp#L993
You should try hooking the "menuselect" command, check if player has a newmenu open and then work out if the player has pressed next or back. Keep in mind that MENU_EXIT and MENU_BACK are special values defined by the newmenu system, so you have to take the number of items and pagination into account.
May I ask what you want to achieve? Maybe there is a more appropriate solution to your problem.
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