Prajakta is correct, you can select microflow as the source for selectable objects in the reference selector properties and pass in your selectable objects from a microflow.
If it fits your uses case another useful way to provide the selectable objects is by selecting XPath for your source of selectable objects and using constraints. When Xpath is selected you are able to select constraints from the constraints input field below the xpath window. The modeler will calculate all possible constraints over association and you can select one or many of these constraints to filter your selectable objects.
Xpath with constraints can be a good option as it will most likely perform better than a microflow source and more importantly it will update the available selectable objects if any of the relevant constraints are changed on the page.
This is all detailed in the documentation as well: https://docs.mendix.com/refguide/reference-selector/
Hopefully this helps you out