@leighrubin Glad that I could help.
I can think of some ideas instead for the code:
If the participants have some way to sign-in, you could probably take them right to the latest survey they're recruited for. This way, they sign in once, and it saves the user (typical to most apps today). This however leads to a different set of problems - what if they've been recruited for multiple surveys and they haven't checked any?
Other than that, the referring link could have the unique code right in the url. When the user taps it, the code is transferred to the app automatically, eliminating the need to type it in. This could also be coupled with the sign in for other advantages (maintaining records and such).
I'll add to this if I can think of other ways I might've observed.