JSFiddle - React, Tailwind, and code Playground

HTML

<form name="sendfrak"  />
  

     <h3>Från</h3>

    <div id='from'>
        <dl><dt>Namn</dt>

            <dd>
                <input type="text" name="fromname" />
            </dd><dt>Adress</dt>

            <dd>
                <input  type="text" name="fromaddr" />
            </dd><dt>Postnr</dt>

            <dd>
                <input type="text" name="frompostnr" />
            </dd><dt>Ort</dt>

            <dd>
                <input type="text" name="fromort" />
            </dd>

            
        </dl>
    </div>
     <h3>Till</h3>

    <div id='rec'>
        <dl><dt>Namn</dt>

            <dd>
                <input type="text" name="toname" />
            </dd><dt>Adress</dt>

            <dd>
                <input type="text" name="toaddr" />
            </dd><dt>Postnr</dt>

            <dd>
                <input type="text" name="topostnr" />
            </dd><dt>Ort</dt>

            <dd>
                <input type="text" name="toort" />
            </dd>
        </dl>
    </div>

JavaScript

$("#from input").blur(function() {
    var value = $(this).val();
    var fromName = $(this).attr('name');
    var toName = fromName.replace('from', 'to');
    
    $('input[name="' + toName + '"]').val(value);
});