I looked at the samples available, the ones about the relationship don't do what we are trying to do. Is there maybe a sample somewhere that will illustrate this?
Look at how the relationship query is in the declarations and read more about it in the API reference to limit the fields etc. The fieldsLayer_selectionCompleteHandler function has all of the code you will need to perform the relationship query, you will have to modify it to your layers and fields. It provides the code for doing the quesry and creating the onResult and onFault coding.
Otherwise you could run a query on the related table and create an array of the selected feature id's and then use that to select the features within your landmark layer.
Look at how the relationship query is in the declarations and read more about it in the API reference to limit the fields etc. The fieldsLayer_selectionCompleteHandler function has all of the code you will need to perform the relationship query, you will have to modify it to your layers and fields. It provides the code for doing the quesry and creating the onResult and onFault coding.
Otherwise you could run a query on the related table and create an array of the selected feature id's and then use that to select the features within your landmark layer.