To make the Subject line custom, you can create a Data Connection --> Submit --> Email Message.
In the Subject line, change the value to be "concat("Generic Text", Field, "generic text", Field) <-- This is just an example on how you can have plain text and multiple fields in the Subject Line. All of the fields in the Data Connection are able to populate with Fields instead of static text.
then on the Button, Add a rule that Submits using a Data Connection and choose that connection you created. You will also have to create a "Main Submit" connection so it knows to update the SharePoint List.
let me know if you need more clarification.