If the intent is to create Contacts module record by accepting (firstname, lastname, email) from the request parameter and respond back with record id then API definition for the same would be as shown below.
You can create a record and also link it parent record using link node.
In the below example you will see label in link node, this is the relationship name(label you see in related list in summary page) between newly created and parent record. If you are not sure then you can leave it skip mentioning it.
Use basic authentication to invoke custom app's api. Below is one example for connecting atlassian api.
You can directly give email and api key in username and password or if you want to take these details from user and store in vtiger then access them using VTAP data api's.