Hi @GaryBritton,is this the same question as the one you asked here?
If so I believe the following may work.
Could you not just have the app record when the introducer sends a code to someone and when that same email is used to login that introducers code is automatically applied?
So workflow would look something like…
Introducer logs [123@gmail.com] as someone they referred.
Days later [123@gmail.com] logs in and the welcome screen asks for name and shows an auto populated non editable text display of the referers code