Use a dynamic stamp.
How do you add the text box - there are a number of different ways to do this in Acrobat, so please be specific.
I would a add this text using the "Add text comment" function on the "Comment" pane in Acrobat XI, or the "Text" function on the "Comment" toolbar in Acrobat DC. This will also add an annotation (the stamp itself is an annotation), and here the order in which annotations are added will define what is in front or behind other objects, so if you add the text later, it will be on top of the stamp.
If you want to add both pieces of information at the same time, then Bernd's suggestion of using a custom dynamic stamp is the better solution. See here for more information: https://acrobatusers.com/tutorials/dynamic_stamp_secrets
I am using a dynamic stamp. The reference ! number changes for each document i process, so the basic info on the dynamic stamp stays the same but I need to add the additional info every time.
I added a feature " add text" to the quick menu at the top. That feature was always placing text behind if I moved it over the stamp. Using a text box or adding a comment seams to work. Thank You.