You need to configure the “Identify & Authorize Application” under “Identify & Access” stage (Refer the screenshot below). API Gateway supports variety of identification/authentication types like HTTP Basic Authentication, API Key, JWT, Oauth, OpenID etc.
From my understanding of your scenario, you can choose “HTTP Basic Authentication” to authenticate the userid/password from the client. The users need to be already configured in API Gateway or you can authenticate against the LDAP users if you have configured LDAP in the API Gateway.
Based on the “Application Lookup condition” you define, you might need to create an application and define the username in the application and register the application to the API. Request you to go through the documentation if you need more information on “Application Lookup condition”.
Hope this answers your question.