In the interface Sparkpost no send button. This service is for sending emails with the API.
You have to make an API request to the service to indicate that the send (template id) and who to send (the id of the mailing list).
Here is the API documentation of sparkota: https://developers.sparkpost.com/api/transmissions
From Sparkpost is integration with Postman (https://www.getpostman.com/)
is a special service for constructing API requests. That's just the Postman can be an interface to Sparkpost.