You'll need to bind the row id value of the selected row of the service that provides the data to your grid.
Look at the service that provides data to the grid, under Returned Data, you have two data sets, All Data and Selected Row. All data will be what you have bound to your grid. Selected row is populated when a row of the grid is selected. As you need specific values from the seleced row click on the + to expand it, use drag and drop as normal to bind the row id to your other service parameter.