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?

JavaScript Solution

Option 1: You can use OnChange of the dropdown widget (if applicable) to call nanoflow which should call JavaScript action.

 In the JavaScript action, you can get the selected item from the dropdown. https://stackoverflow.com/questions/1085801/get-selected-value-in-dropdown-list-using-javascript

Option 2: you can add HTML Snippet widget from marketplace and write your JavaScript event listener to implement the same

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