Invoke WebAPI through an external Java Web Application. How we can authenticate logged in user?

Hello All,

Our customer has an onsite Appian 17.2 environment in which the users are authenticated based on their LDAP credentials. We are assigned to present a Task to users from an external web application. We are planning to use the Embedded Sail interface feature. Therefore we have enabled and configured SAML with the LDAP details in order to enable the SSO required for loading an Embedded sail interface in an external web environment. We have also implemented a WebAPI that starts a new process instance and returns back the task id in order to be used as a parameter for the embedded sail. The issue that we are facing has to do with the authentication of the logged in user before the invoke of the WebAPI. We need to authenticate the logged in windows user in order for the WebAPI to be invoked with the logged in user account and set him as the process initiator and not with a generic user account. Can you propose some ideas on how we can authenticate a user in a Java web application (Java servlet) outside of an Appian SSO enabled onsite environment?

Thanks in advance