In Mendix you can use a reference selector for this and than add values to your drop down on several ways. In my case I have an House which has an housekeeper. I only want to show the available persons there. I add 2 entities, House and Person and make an Association between those 2 named HouseKeeper.
Now on the detail page of House I add a reference selector from the toolbox or drag and drop the Name property on the page
Then open the tab "Selectable objects" and you have 3 options, in my case i select xpath datasource and give it an xpath constraint '[Available]' which equals 'Available = true()'.
More info on the other options here: