So you have some entity (say ‘Invoice’) for which there is a form/data view (say ‘Invoice_NewEdit’).
In that form you have a reference selector to set association with another entity (say ‘Client’), with a microflow as onchange event:
Since the microflow is onchange for an Invoice, it will receive an ‘Invoice’ object as parameter.
Now you can just retrieve the associated object over the same association the reference selector uses (in my example to simply log the name of the Client that you just selected):
Hope this answers your question,