you can't use a dropdown field with mutiple value because it was made to handle single value. as the checkbox and radio this is why there are checkboxList(i guess).
But i found a solution.
Use a partial as field type in fields.yaml instead of dropdown
https://octobercms.com/docs/backend/forms#field-partial
Create a partial with the content below (notice the [] in name attribute this is why it work !)
But be aware that this is just a trick and you can only use direct yaml options assignment https://octobercms.com/docs/backend/forms#field-dropdown
<?php
$fieldOptions = $field->options();
//get the field value as an array
$selectedValues = (array)$field->value;
?>
<!-- Dropdown -->
<?php if ($this->previewMode): ?>
<div class="form-control"><?= (isset($fieldOptions[$field->value])) ? e(trans($fieldOptions[$field->value])) : '' ?></div>
<?php else: ?>
<select
id="<?= $field->getId() ?>"
name="<?= $field->getName() ?>[]"
class="form-control custom-select"
<?= $field->getAttributes() ?>>
<?php if ($field->placeholder): ?>
<option value=""><?= e(trans($field->placeholder)) ?></option>
<?php endif ?>
<?php foreach ($fieldOptions as $value => $option): ?>
<?php
if (!is_array($option)) $option = [$option];
?>
<option
<?= in_array($value, $selectedValues) ? 'selected="selected"' : '' ?>
<?php if (isset($option[1])): ?>data-<?=strpos($option[1],'.')?'image':'icon'?>="<?= $option[1] ?>"<?php endif ?>
value="<?= $value ?>">
<?= e(trans($option[0])) ?>
</option>
<?php endforeach ?>
</select>
<?php endif?>
And for the yaml
```
select_field:
label: Sample
type: partial
path:$/author/plugin/models/classfolder/_my_partial.htm
attributes: {multiple:'multiple'}
options:
key:value
key:value
```
A better approche would be may be to build a widget or make a pull request
if you have the ability you can touch the core and add
the same content in \modules\backend\widgets\form\partials
with name _field_dropdownlist.htm
then in \modules\backend\widgets\form\Form.php line 630
change :
$optionModelTypes = ['dropdown', 'radio', 'checkboxlist', 'balloon-selector'];
to add your partial name without _field or .htm ex _field_dropdownlist.htm become dropdowList
$optionModelTypes = ['dropdown', 'radio', 'checkboxlist', 'balloon-selector','dropdowlist'];
now in your yaml file juste use type:dropdownList and it will work.