I need fundamental understanding of panel

I need to set flag = SHOW later when panel already defined?.

and how to insert button in panel when created this way?.

function call_MainMenu()
{
PANEL* MainMenu = pan_create("flags=SHOW",999); //create a panel at 0,0 at runtime
BMAP* bmpMainMenu = "mainmenu.pcx"; //Set the bitmap

MainMenu.bmap = bmpMainMenu; // NOW add the bmap to the Panel
MainMenu.pos_x = (screen_size.x - bmap_width(bmpMainMenu)) / 2;

MainMenu.pos_y = 50;
// Need buttons here
}