Mendix Widget Solution:
Are you using custom dropdown for this? if Mendix provided dropdown, adding on change event to microflow will have the entity as parameter.
If you are using custom dropdown developed by you, I can help on this. or are you using any marketplace widget for this?
you have option onchange in refrence selector here directly call a microflow
to get your selected data into microflow as a parameter add entity paramter in microflow and you can use attribute value there
example : $entityobject/attributeSelectedInRefSelector !=empty