SendinBlue — is a free service of email marketing, SMS, transactional email (SMTP). To send the data from forms on your website to SendinBlue contact list you need to fill out on Tilda your SendingBlue account API key.
Choose your profile and the point SMTP&API in the SendinBlue account.
Copy API key (version v3)
Go to your account on Tilda, Site Settings → Forms → SendinBlue
Put the API key and save settings.
Choose contact list from existent ones. If you need a new list you should create it before.
If you need to send data to different lists, you should assign SendingBlue service several times.
You can assign the service to all forms on the website, by clicking 'Assign the service to all forms on the website'
Go back to the page, add the block with the form and open the content settings menu. Tick the box 'SendinBlue'. Publish the page.