ctrlMapScreenToWorld

You can get the screen coordinates by the UI Event Handlers onMouseButtonDown, onMouseButtonUp, onMouseButtonClick, onMouseButtonDblClick.

The return Array is in 2D, you can use it with all set-position commands.

_x = returnArray select 0;
_y = returnArray select 1;

The command parameters are screen position coordinates, which may not equate to the map control's coordinates. A map control's screen coordinates and size can be found use the ctrlPosition command. This is an issue when using the Arma 3 in-game map, which is not fullscreen (all previous titles used full screen map controls, so map control coords did equate to screen coords).

This example creates a RscMapControl and hints you the [x,y] position where you clicked.

0 spawn {
    disableSerialization;
    _map = findDisplay 46 createDisplay "RscCredits" ctrlCreate ["RscMapControl", -1];
    _map ctrlSetPosition [0,0,1,1];
    _map ctrlCommit 0;

    _map ctrlAddEventHandler ["mouseButtonDown", {
        _ctrl = _this select 0;
        _x = _this select 2;
        _y = _this select 3;

        _pos = _ctrl ctrlMapScreenToWorld [_x, _y];
        hint format ["pos: %1", _pos];
    }];
};