$fieldset->addField( 'first_name', 'text', [ 'name' => 'FirstName', 'label' => __('First Name'), 'id' => 'FirstName', 'title' => __('First Name'), 'required' => true, 'class' => 'input-text', 'data-validate' => '{"required":true, "validate-FirstName":true}', 'value' => $firstname // Set default value ] );