PHP & Sendinblue expert

I have a PHP form in my landing page that works fine: You fill the info, click on register and submit. I receive the registration info in my email client, fine. But I need that, regardless I get the email, I want sendinblue to grab the info, (not just relay thru the SMTP), add the contact to my contacts list and automatically trigger my "Welcome Email" workflow (that I have already configured and it works if I reply from my email client).

In short: sendinblue to:
- grab the contact info as it goes thru SMTP,
- add to contacts list and
- trigger Welcome Email Workflow.

Attached my send_form_email.php and my HTML form sample page.