Implementing Stripe hosted checkout

Hi,

We have a requirement to implement the stripe hosted checkout for our application.

To get a callback from the stripe once the checkout session has completed, We have created a webapi in appian and provided the endpoint in the stripe's webhook dashboard so that once the checkout session is completed Appian webapi is called. It seems like we cannot pass the API key for the appian webapi as a header when the stripe invokes the appian webapi so the authentication doesnot happen. The webapi works fine when triggered with Postman. 

Is there any way we can resolve this issue?