Hi Bobby,
Where do you put the API key? You need to put your API Key on the api-key Header’s Value.
This video made by Victor can help you :
And after setting these things correctly you will still get a error that like Your SMTP account is not yet activated.
Then you need to contact Sendinblue to activate that for you! Here’s a topic created by Santiago : Sendinblue: Your SMTP account is not yet activated
Thank you