When mapping the Read operation in a Data Model, you can map input, output, and changing parameters of the Properties.
Your must include the constant values that you require.
If Screen Scraping is your data source, get a search help (Query) entity from a recording that includes the Read operation.
In addition, you can use the SAP Gateway Generators Cookbook to quickly create a Data Model and test the generated SAP Gateway service using the Data Model Generator tool.