Yes I have checked it, for now we cannot you with that as the addon doesn't have the functionality that you are looking for. 

You should use the Dropdown with its functionality for now.

Please double-check that the field type is "Dropdown".

But I really would LOVE to have the ability to control who can see custom fields. In fact I had this topic opened asking this same question: https://gvectors.com/forum/wpforo-topic-custom-fields/readonly-vs-editable-custom-fields-based-on-user-role/

I'd recommend the following change in the topic custom fields plugin until we release a new version.

Use some plugin which allows editing plugin files or the built-in WordPress plugin file editor, find and open this file: wp-content/plugins/wpforo-topic-custom-fields/wpforotcf.php

Find this code fragment:

private function field_values_to_string($field){
    if( $values = wpfval($field, 'values') ){
        if( is_array($values) ) {
                  if( $field === 'select' ){
                      $string = '';
                      foreach( $field as $k => $v ){
                          if( is_array($v) ){
                           $string .= "\n\n" . implode( "\n", $v ) . "\n";
                          }else{
                              $string .= "\n$v";
                          }
                      }
                   $field = trim($string);
                  }else{
                   $field = implode("\n", $values);
                  }
              }
          }
          return $field;
}

Replace to this and save:

private function field_values_to_array($field){
   if( $values = wpfval($field, 'values') ){
      if( is_scalar($values) ) {
         $field = explode( "\n", $values );
                  if( $field === 'select' ){
                   $v = [];
                   $optgroup = '';
                   foreach( $field as $value ){
                    if( preg_match( '#^ *\[optgroup *= *(]+?) *] *$#iu', $value, $m ) ){
                       $optgroup = $m;
                       $v = [];
                    }elseif( preg_match( '#^ *\ *$#iu', $value, $m ) ){
                       $optgroup = '';
                    }else{
                       if( $optgroup ){
                          $v[] = $value;
                       }else{
                          $v[] = $value;
                       }
                    }
                   }
                   $field = $v;
                  }
      }
   }
   return $field;
}

Thanks for the follow up.

I copied the syntax that was included in the screenshot but unfortunately it didn't work.

Could you please try this syntax for dropdown options:

Value 1
Value 2


G2 Value 1
G2 Value 2

Yes, we'll take this under consideration for sure.