Base64Decode and FMP URL Parameters

I'm on iOS. I'm using iOS shortcuts and FM Go.

In FM Go, I send a container's PDF to iOS shortcuts via a x-callback-url with an "input" of the Base64Encode PDF. I annotate the PDF in iOS Shortcuts. I then send the annotated PDF back to FM Go to insert it back into the container...however there seems to be some characters in the Base64encode fmp url parameter that can't be passed in the URL coming back.

Is there something I can do to further encode this Base64 script parameter (that is the annotated PDF) so that it will pass back to FM Go via the URL.