use prawee\widgets\ButtonAjax;
use yii\bootstrap\Modal;

echo ButtonAjax::widget([
    'name'=>'Create',
    'route'=>['create'],
    'modalId'=>'#main-modal',
    'modalContent'=>'#main-content-modal',
    'options'=>[
        'class'=>'btn btn-success',
        'title'=>'Button for create application',
    ]
]);
...
Modal::begin(['id'=>'main-modal']);
echo '<div id="main-content-modal"></div>';
Modal::end();

public functionn actionCreate(){
    ...
    if(Yii::$app->getRequest()->isAjax){
        return $this->renderAjax('create', [
            'model' => $model,
        ]);
    }else{
        return $this->render('create', [
            'model' => $model,
        ]);
    }
}