The ACSUtils cursor library allows mods to easily add mouse cursor support.

script "Menu" (void) CLIENTSIDE
{
    while (true)
    {
        UpdateCursor();
        // Draw cursor at CursorX(), CursorY()
        Delay(1);
    }
}

UpdateCursor reads player input and moves the cursor. CursorX and CursorY return the current position of the cursor on the screen in HUD coordinate system.