Private Sub Worksheet_BeforeDoubleClick(ByVal Target As Range, Cancel As Boolean)

  If Not Application.Intersect(Target, Range("AG4:AG910")) Is Nothing Then
  Dim FileNames As Variant
  Dim Msg As String
  Dim i As Integer
  FileNames = Application.GetOpenFilename(MultiSelect:=True)
  If IsArray(FileNames) Then
      For i = LBound(FileNames) To UBound(FileNames)
          Msg = Msg & FileNames(i) & vbNewLine
      Next i
      Target = Msg
          Else
      MsgBox "No files were selected."
End If

End If

function onOpen() {

  SpreadsheetApp.getUi()
      .createMenu('Custom Menu')
      .addItem('Show Upload Dialog', 'showUploadBox')
      .addToUi();
};

function showUploadBox() {

  var htmlOutput = HtmlService.createHtmlOutputFromFile('Dialog')
     .setWidth(500)
     .setHeight(500);

  SpreadsheetApp.getUi().showModalDialog(htmlOutput, 'Title of Dialog');
};

<div id="formDiv">

<form id="myForm">

    <input id="fileName" name="picToLoad" type="file" />
    <br>
    <br/>
    <input type="button" value="Submit" onclick="fncGetFileName()" />
  <br>
  <br>
  <br>
    <input id="idBtnClose" type="button" value="Close" style="display: none" onclick="google.script.host.close()" />
</form>
</div>

<br>
<br>

<div id="status" style="display: none">
  <!-- div will be filled with innerHTML after form submission. -->
  Working. Please wait...
</div>


<script>

function fncGetFileName(frmData) {
  console.log('fncGetFileName ran!');

  var theFileName = document.getElementById('fileName').value;
  theFileName = theFileName.slice(12);

  console.log('theFileName: ' + theFileName);

  document.getElementById('status').style.display = 'inline'; //Display msg

  google.script.run
    .withSuccessHandler(updateOutput)
    .processForm(theFileName)
};
  // Javascript function called by "submit" button handler,
  // to show results.

  function updateOutput() {

    var outputDiv = document.getElementById('status');
    outputDiv.innerHTML = "The File Name was Written!";

    document.getElementById('idBtnClose').style.display = 'inline'; //Display msg
  }

</script>

function processForm(argFileName) {
  Logger.log('argFileName: ' + argFileName);

  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var theSheet = ss.getActiveSheet();

  var theRange = theSheet.getRange("B4");
  theRange.setValue(argFileName);
};