//specify path to your external page:
var iframesrc="external_missions.htm"

//You may change most attributes of iframe tag below, such as width and height:
document.write('<iframe id="datamain" src="'+iframesrc+'" width="290px" height="120px" marginwidth="0" marginheight="0" hspace="0" vspace="0" frameborder="0" scrolling="no"></iframe>')