<?= $form->field($model, 'employee_id')->dropDownlist(
                 ArrayHelper::map(Employee::find()->all(), 'id', 'employee_name'),
                 [
                  'prompt' => 'Select Employee ID',
                  'style' => 'width:200px'
          ]); ?>

Employee::find([
    'employee_status' => 'Available'
])->all();