How to attach XML code to PDF

Let's take a step back: What exactly are you trying to do? Do you want to apply a highlight annotation, or do you want to provide a temporary highlight the way it's done when you search for information in your document?

My next question is why do you want to use XML? Do you already have an existing XML based workflow?

Where does information about the data you want to highlight come from? And, how do you want to import that into Acrobat?

To see what you can do, please perform the following steps:

Open a PDF file with text in it, and add some text highlight annotations. Then select the "..." menu on the comments pane on the right side, and pick "Export All to Data File..." and select the "XFDF" output format. This now contains information about your highlight annotations in an XML file. You can delete your annotations in your document, and then select to "Import Data File..." from the same menu, select your just exported file and see how your highlight annotations are getting restored.

All you need to do is to mimic the format in the XFDF file. When it comes to analyzing what data needs to go where, you are unfortunately on your own.