Google Sheets script to allow multi-select in cells with data-validation (adapted from https://www.youtube.com/watch?v=dm4z9l26O0I)

dialog.html

<div style="font-family: sans-serif;">
<? var data = valid(); ?>
<form id="form" name="form">
<? if(Object.prototype.toString.call(data) === '[object Array]') { ?>
<? for (var i = 0; i < data.length; i++) { ?>
<? for (var j = 0; j < data[i].length; j++) { ?>
<input type="checkbox" id="ch<?= '' + i + j ?>" name="ch<?= '' + i + j ?>" value="<?= data[i][j] ?>"><?= data[i][j] ?><br>
<? } ?>
<? } ?>
<? } else { ?>
<p>This cell has no <a href="https://support.google.com/drive/answer/139705?hl=en">Data validation</a>.</p>
<? } ?>
<input type="button" value="Select" onclick="google.script.run.fillCell(this.parentNode)" />
<input type="button" value="Refresh validation" onclick="google.script.run.showDialog()" />
</form>
</div>

multi-select.gs

function onOpen(e) {
  SpreadsheetApp.getUi()
  .createMenu('Scripts')
  .addItem('Multi-select for this cell...', 'showDialog')
  .addToUi();
}
function showDialog() {  
  var html = HtmlService.createTemplateFromFile('dialog').evaluate();
  SpreadsheetApp.getUi()
  .showSidebar(html);
}
var valid = function(){
  try{
    return SpreadsheetApp.getActiveRange().getDataValidation().getCriteriaValues()[0].getValues();
  }catch(e){
    return null
  }
}
function fillCell(e){
  var s = [];
  for(var i in e){
    if(i.substr(0, 2) == 'ch') s.push(e[i]);
  }
  if(s.length) SpreadsheetApp.getActiveRange().setValue(s.join(', '));
}