RichTextBox hyperlinks strange behavior...

Here's the issue:

Example: If I open an rtf file in Notepad and insert the following string somewhere,

{\field{\*\fldinst { HYPERLINK "TEST LINK" }}{\fldrslt {TEST}}}

save the file (as .rtf) and then load it into a RichTextBox, the link shows up as a clickable, underlined link. Great. This is what I want, a hyperlink with a non-standard link type (i.e. not one of the "standard" link types like file:/// or http:/// etc., just a plain old text link.)

HOWEVER, trying to insert the link directly into the richtextbox at runtime, like this:
(assume the selection is not nothing)

SelectedRtf="\rtf1\ansi {\field{\*\fldinst { HYPERLINK ""TEST LINK"" }}{\fldrslt {TEST}}}"

Fails to modify the rtf. Why is this?