Including a multiple select field in my module using data from a database field

You can add a multiple select field to your form by defining its properties and also including the field in your list of add or edit fields.

Here's a sample code to include a multiple select field to your form by using data from a database field;  

 
$objConfig->multipleSelect('field_name', 'field_label', $required = 0)
$objConfig->childDatabase('table_name', 'id_field', 'value_field');