DOM Event – Where Does It Go?

I want to use a DOM function to copy the contents of one field to another field before submission. The code I wrote is:

document.addEventListener( 'wpcf7submit', function( event ) {
     var homePhone = document.getElementById("CellNumber").value;
     document.getElementById("HomeNumber").value = homePhone;
}, false );

Two questions. 1.) Does this look correct? 2.) Where does the code go? In the functions.php file or the Additional Settings tab of the form, or somewhere else?

Thanks.

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