You rarely use "SendAsync" directly when you work with the SDK as there are C# methods available for all engine API endpoints. In your case you want to use the "CreateConnection" endpoint in this interface:
I can't really help you there, but what I typically do when I try to figure out what settings to use is to look at the websocket traffic in the browser when I do the operation there. So if you know how to create the connection through Qlik Sense, then open the developer tools in Chrome and check what the message looks like in the websocket traffic when you create it. You should be able to find the call to "CreateConnection" there.
I went through the Document link based on which i have rendered the code as mentioned below but on execution iam able to create an app but iam failing to create the rest data source with invalid parameters error