trigger button.onclick event from onbeforeunload

i am currently developing an asp.net project. there is a previous, next, and cancel button that the user can utilize, but the problem is that everything is broken into controls so that if they hit the browser's back button it will reset everything and take them to the very beginning. i would like to capture the onbeforeunload event and trigger the previous button click event (i.e. treating navigation like clicking the previous button).
 
Any help is extremely appreciated for this has been truly aggravating.
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