I would like to add a shortcode to the success message

I would like to add a shortcode to the success message

i.e I have modified the success message to include a short code.
“Thank you for your message. It has been sent. please send money [qpp] to me.”

The system returns the characters rather than the executing the short code.
See: https://dev.they-working.org/webmaster-feedback-contact/

however when I do the same thing in Ninja-Forms it works.
See: https://dev.they-working.org/webmaster-feedback-ninja/

Any thoughts how I can make it work in contact?

The page I need help with: [log in to see the link]