Tilføj tekst til textarea fra dropdown

function addToList2(dropdown, outputObjID) {
  var selectedValue = dropdown.options[dropdown.selectedIndex].value;
  document.getElementById(outputObjID).value += selectedValue + "\r\n";
}

document.getElementById(outputObjID).value = valueList.join('\r\n');

document.getElementById(outputObjID).value += valueList.join('\r\n');

<script type="text/javascript">
      var selectionHasChanged = function() {
        // det samlede indhold
        var output = []

        // hent indholdet af service checkboxes
        var services = document.getElementsByName('service[]');
        for(var i = 0; i < services.length; i++) {
          if ( services[i].checked ) {
            output.push(services[i].value);
          }
        }

        // hent indholdet af dropdown
        var dropdown = document.getElementsByName('antal');
        if (dropdown && dropdown.selectedIndex > 0) {
          output.push(dropdown.options[dropdown.selectedIndex].value);
        }

        // saml alt indholde
        document.getElementById('txt1').value = output.join("\n");
      }
    </script>

onClick="addToList(this, 'txt1')"

onClick="selectionHasChanged()"

<select name="antal" onchange="selectionHasChanged()">
  <option>Vælg antal ...</option>
  <option value="1">1</option>
  <option value="2">2</option>
 </select>

<script type="text/javascript">
      var selectionHasChanged = function() {
        // det samlede indhold
        var output = []

        // hent indholdet af service checkboxes
        var services = document.getElementsByName('service[]');
        for(var i = 0; i < services.length; i++) {
          if ( services[i].checked ) {
            output.push(services[i].value);
          }
        }

        // hent indholdet af dropdown
        var dropdown = document.getElementById('antal');
        if (dropdown && dropdown.selectedIndex > 0) {
          output.push(dropdown.options[dropdown.selectedIndex].value);
        }

        // saml alt indholde
        document.getElementById('txt1').value = output.join("\n");
      }
    </script>

<select id="antal" onchange="selectionHasChanged()">
  <option>Vælg antal ...</option>
  <option value="1">1</option>
  <option value="2">2</option>
 </select>

<script type="text/javascript">
  var selectionHasChanged = function() {
    // det samlede indhold
    var output = []

    // hent indholdet af service checkboxes
    var services = document.getElementsByName('service');
    for(var i = 0; i < services.length; i++) {
      var service = services[i];

      switch(service.type) {
        case 'checkbox':
        if ( service.checked ) {
          output.push(service.value);
        }
        break;

        case 'select-one':
        if (service.selectedIndex > 0) {
          output.push(service.options[service.selectedIndex].value);
        }
        break;
      }
    }

    // saml alt indhold
    document.getElementById('txt1').value = output.join("\n");
  }
</script>

<input type="checkbox" name="service" onchange="selectionHasChanged()" value="value">

<select name="service" onchange="selectionHasChanged()">