New action button for custom form

Questions
1

I am trying to add a custom button that pass some info to a form that once submitted generate an invoice in a form of pdf.

I have added the following into …app/Template/project_header/views.php

	<li>
        <?= $this->url->icon('file-pdf-o', t('Invoice'), 'InvoiceController', 'create', array('project_id' => $project['id'], 'search' => $filters['search']), false, 'view-listing', t('Keyboard shortcut: "%s"', 'v i')) ?>
    </li>

Then once I click that button I would need to generate a form through the custom controller with the method create()

    public function create()
    {
    	$status = array(TaskModel::STATUS_OPEN);
    	
    	$project = $this->getProject();
    	$task_ids = $this->db
        ->table(TaskModel::TABLE)
        ->eq(TaskModel::TABLE.'.project_id', $project['id'])
        ->in(TaskModel::TABLE.'.is_active', $status)
        ->asc(TaskModel::TABLE.'.id')
        ->findAllByColumn(TaskModel::TABLE.'.id');

        $this->response->html($this->template->render('invoice/create', array(
        	'project' 	  => $project,
        	'values' => array(
        		'invRef'	  => $project['name'],
            	'invFrom'     => implode(',', array('My Company','Address line 1','Address line 2','City and zip','Country','VAT number')),
            	'invTo'       => implode(',', array('My Client','Address line 1','Address line 2','City and zip','Country','VAT number')),
        		'invDate'     => '',
        		'invDue'      => '',
        		'invAdd'      => implode(',',$task_ids),
            )
        ))); 
    }

That should populate the below form:

<div class="page-header">
    <h2><?= 'Invoice for Project: '. $project['name'] ?></h2>
</div>
<form action="<?= $this->url->href('InvoiceController', 'create', array('project_id' => $project['id'])) ?>" method="post" autocomplete="off">
	<?= $this->form->csrf() ?>
	<?= $this->form->label(t('Reference'), 'invRef') ?>
	<?= $this->form->text('invRef', $values, array('required')) ?>
	<br><br>
	<?= $this->form->label(t('From'), 'invFrom') ?>
	<?= $this->form->text('invFrom', $values,  array('required')) ?>
	<br><br>
	<?= $this->form->label(t('To'), 'invTo') ?>
	<?= $this->form->text('invTo', $values, array('required')) ?>
	<br><br>
	<?= $this->form->label(t('Date'), 'invDate') ?>
	<?= $this->form->text('invDate', $values, array('required')) ?>
	<br><br>
	<?= $this->form->label(t('Due'), 'invDue') ?>
	<?= $this->form->text('invDue', $values, array('required')) ?>
	<br><br>
	<?= $this->form->label(t('Add'), 'invAdd') ?>
	<?= $this->form->text('invAdd', $values, array('required')) ?>
    <?= $this->modal->submitButtons() ?>
</form>

However, the form does not appear as a modal but a brand new webpage, which is not what I would like to have. The point is to have a form with pre-filled info that I can fill to generate an invoice, based on the project and related tasks.

I am quite n000b with PHP and I would love your help here.

Best,

Teo