连接Android - VB.Net应用程序，在本地服务器上运行

Please bear with me as I am new to all this.

I have to build an application in VB.Net that connects with another popular Windows Application also in VB.NET which we will call AppB. The thing is I have done the integration of my VB application to AppB but I am not sure how should I connect myApp (my vb application) to the backend?

The flow is as follows

1. User clicks a button on Android app
2. this will send Android app data to myApp via backend
3. myApp connects and sends data to AppB
4. AppB calculates data
5. AppB sends response back to myApp
6. myApp sends data back to Android via backend

Basically, Android will invoke the backend to ask something from myApp which will eventually ask the same thing from AppB. AppB will return the response to myApp which will send it back to Android via the backend.

How can I connect the backend to AppB written in VB.NET? I was able to setup SOAP (Web Reference) from myApp to a PHP backend but I don't know how to reverse it, meaning how can I call myApp from the backend?

Thanks in Advance.