How to add multiple annotations to pdf document in box

Hello , I have the following problem , I wonder , how can I connect annotation to the selected box , I need the following functionality : I create pdf with embedded 3d annotation and then I select the region(area)  in the pdf doc where annotation should be added. I have the following code from samples that sets the box for annotation .How can I add multiple annotations to the page and activate them immediately after selecting region for my annotation.

ASFixedRect annotBox = {ASInt32ToFixed(108), ASInt32ToFixed(612 - 108),

   ASInt32ToFixed(792 - 108), ASInt32ToFixed(108)};

theAnnot = PDPageAddNewAnnot(pdPage, -2, ASAtomFromString("U3D"), &annotBox);

  PDAnnotSetFlags(theAnnot, pdAnnotPrint | pdAnnotReadOnly);