<?= $form->field($model, 'category_id')->dropDownList(ArrayHelper::map(Category::find()->all(), 'id','name'),['style'=>'font-family: \'Exo 2\', sans-serif; color: #0f0f0f; font-size:17px'])->label('Отделение')?>
<?= $form->field($model, 'category_id')->dropDownList(['' => 'Укажите отделение'] + ArrayHelper::map(Category::find()->all(), 'id','name'),['style'=>'font-family: \'Exo 2\', sans-serif; color: #0f0f0f; font-size:17px'])->label('Отделение')?>