Sorry, but I am really struggling to understand what you want.

What do you mean by coupon code which admin given

YOU decide what Coupon Code to apply. YOU will choose a name for your coupon AND the CODE.

You could, for example, choose:

COUPON NAME: Christmas Specials
COUPON CODE: XMAS2012

If you want customers / visitors to the site to be told the code, then just put it into a sidebox, a banner, or even in a bit of HTML in your define_main_page.php