You will need to add custom Javasript on the change of your option set or on Save of Form. Get the option set Text and append it with the Subject Field.
var optionset = Xrm.Page.getAttribute("attributename")
var optionText= optionset.getText();
var subjectText = Xrm.Page.getAttribute("subject").getValue();
Xrm.Page.getAttribute("subject").setValue( subjectText +" "+ optionText);
please also put the null values checks .