// create the SData client
var client = new SDataClient("http://localhost:3333/sdata/slx/dynamic/-/")
{
    UserName = "admin",
    Password = ""
};

// first create a new/empty salesprocess record
var salesProcess = client.Post(new {}, "salesProcesses");

// now call the InitSalesProcess business rule to start the process and
// attach it to the opportunity. We'll pass into the rule the new
// salesprocess entity we created along with the plugin ID of the
// process and the opportunity ID we're attaching it to.

var processPluginId = "p6UJ9A000501"; // ID of the process in the plugin table
var opportunityId = "ODEMOA0000CR"; // ID of the oppportunity we're attaching it to

// create the payload for the business rule
var initProcessRule = new
{
    Request = new
    {
        entity = salesProcess,
        pluginId = processPluginId,
        entityId = opportunityId
    }
};

// execute the rule
client.Post(initProcessRule, "salesProcesses/$service/InitSalesProcess");