What are different ways to run the client application to verify the response without having server?

I have a CPP client application. As part of client application, I will send a message in encrypt format to http server which will available as part of another server application.

Currently, server application is not yet implemented. But I want to test the client application by running it and I want to verify the response.

What are different ways to run the client application to verify the response without having server? Please help me.

Thanks.