Web API exposing start process smart service gives 404 Error

Hi all,

I have created a Web API exposing start process smart service.
Eg: An Email or attachment(containing Web Api endpoint) is sent to a Manager group from Appian, on its click it should
redirect to Appian and a process instance to be initiated.

When I test the Web API from Appian the process model is getting triggered whereas on Click on link sent in Email, it shows 404 resource not found error.

Added Viewer rights as well for the user.
In Process-model, I have checked the option - Allow anyone to fire triggers.

How shall I configure Authentication in this case?

Any help would be appreciated.

Thanks and Regards