關於警告視窗...的做法

各位前輩好~~下面是一個下拉式選單的網頁我想再裡面加入一個"彈跳出警告視窗"~~因為怕使用者不選擇就直接按GO去連結請問該怎麼做PS:在下是網頁新手~~若問題深度不夠~~還請大家多多包含
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