When creating artwork for specialty inks, it's best practice to separate the specialty ink from the rest of the artwork by creating a custom layer named "Special Ink." You'll also want to create a custom spot-colors swatch and name it the same way as the layer, "Special Ink." You can make the spot color any color you'd like, although you should make it much different than the background artwork and text to ensure that it is noticeable against the base artwork.
When you export the file to a Press Ready PDF in InDesign, open the output section in the PDF export screen, select Ink Manager, and ensure that the custom spot color "Specialty Ink" is exporting as a spot (and not process) color. Illustrator will automatically save the spot color to a PDF when you select Save As PDF.