@ddiehl,
You can get the rectangular coordinates of a TextBoxField, and then add a hyperlink to that region of the PDF page. Please try the following code to retrieve the rectangle coordinates:
[VB]
Dim pdfDocument As New Document("C:\Pdf\test249\TextBox.pdf")
Dim textBoxField As TextBoxField = TryCast(pdfDocument.Form("textbox1"), TextBoxField)
Dim rectangle As Aspose.Pdf.Rectangle = textBoxField.Rect
In order to add a hyperlink in the region of PDF page, please refer to this help topic: Add Hyperlink in a PDF File