Need create POST form in Java project in android studio with PHP backend

Hello,

I have Android studio project made with JAVA. Backend is PHP.
Task is simple post username in application and PHP backend must accept it and send simple reward if username is correct. Send with json. Price is small for simple task, but if you do it corrrect, i have many others jobs in this application. Write in description saparas job 15 USD, this means your read description.
So, you need be good in Java, android studio and PHP.
Regards.