The post which you have referred is to provide a datasource in content fragment by using an already registered servlet. So, for e.g if you have a servlet registered with sling:resourceType as /bin/dropdownList then you can use that on the datasource node by setting sling:resourceType property as /bin/dropdownList but you cannot add parameters to it since it will interrupt your sling resolution.
If you want to get some data from your datasource to your servlet what you can try is setting some extra property on your datasource and set the value which you want to pass e.g dummyProperty and then you can probably retrieve that datasource node in your servlet to fetch it
For e.g
request.getResource().getChild("datasource").getValueMap().get("dummyProperty");I haven't tried this though. But hope this helps.