JSFiddle - React, Tailwind, and code Playground

HTML

<input id="authorities" name="authorities" type="text">
    
<input id="team_authority_emails" name="team[authority_emails]" type="hidden" value="{[email protected],[email protected]}">
    
 <input id="enter" type="submit">

JavaScript

String.prototype.replaceAt=function(index,string,separator)
{console.log(index,separator,string);
 return this.substr(0, index) + separator + string +  +     this.substr(index+string.length);
}

$("#enter").click(function() {
  var inputA = $("#authorities").val();
  var inputB = $("#team_authority_emails").val();
  console.log(inputB.replaceAt((inputB.length-1),inputA));
  $("#team_authority_emails").val(inputB.replaceAt((inputB.length-1),inputA, "," ));
});