Make API call from within ghost theme (or elsewhere)

I have a ghost installation and I am using a custom theme.

I want to be able to trigger a API call when a new member is signed up.
For this I want to find the place where the member is added to the ghost database so that I can insert an api call to add this member to sendgrid (because zapier does not work for sendgrid)

Here is how to add a member to sendgrid:

To add a recipient to your contact lists you would need to make two different API calls. The first would be a POST call to the following endpoint. This call is specifically adding the recipient address to the system. HTTP/1.1
The body of that post call would look something like this.

  {    "email": "",    "last_name": "Jones",    "pet": "Fluffy",    "age": 25  }]

You will simply just need to specify the email address that you are adding, as well as specify any custom fields that you might use.

The second call that will need to be made is to add that recipient to a specific list. You can do this by making a POST call to the following endpoint.{list_id}/recipients/{recipient_id} HTTP/1.1

You will want to make sure that you replace {recipient_id} with the actual recipient ID for the address that you just added in the previous call.