Existing mock utility for mocking a UI application

I am creating an application B that will serve users coming from an other application A. So users navigate on application A, and offer links that will have the user navigate to my application B.

I would need to mock the service A so, that users could define what user they are, with what paremeters, and what they want to do. Application A sends as a context all that information to my application B when pressing the link.

I could create my own mocking UI page where I could define what actions user can make, but are there off-the-shelf options for doing this?