The customer's authentication flow mechanism needs more requests to authorize specific method. They need to create custom form to achieve testing thanks to 'try it' button @ documentation.
Is there any way to add more auth options in drop down list?
 

Portal 3.5

Authentication for Try it out functionality cannot be modified by the customer. 

Authentication methods are limited to None, API Key, OAuth 1.0, and OAuth 2.0. OAuth 2.0 grant types Authorization Code, Implicit, Resource Owner Password Credentials, and Client Credentials are supported. 
HTTP Basic authentication is also supported.