add_filter('dwqa_ask_form_tag_output','dwqa_ask_form_tag_output',10,1);
function dwqa_ask_form_tag_output($output){
$args=array(
'orderby' => 'count',
'order' => 'DESC',
'hide_empty' => false,
);

$tag_terms=get_terms('dwqa-question_tag', $args );
$output='<select required multiple="multiple" name="question-tag"
id="question-tag" class="select-tag">';
$output.='<option value="">Tags to be selected</option>';
foreach($tag_terms as $tag_term)
$output.='<option value="'.$tag_term->name.'">'.$tag_term->name.'</option>';
$output.='</select>';
return $output;
}

$tag_value=isset( $_POST['question-tag'] ) ? esc_html( $_POST['question-tag'] ) : '';
$tag_input='<input type="text" name="question-tag[]" id="question-tag" value="'.$tag_value.'" />';
$tag_input=apply_filters('dwqa_ask_form_tag_output',$tag_input);
echo $tag_input;