I’ll try to help you tomorrow
OK, thank you.I’m waiting for your answer!
Hey, maybe you find this interesting in that regard…
You just remove the possibility to signup without referral code, and the trick is done! 
(1) You could use the trick with single item list. Imagine you have a “Invites” collection, with “Code” property, which is numeric.
On the “please enter the code” screen, you put an Input box and a button. Then convert a button to a list of Invites, and add a filter to the list that Current Invite → Code is equal to Input.
As soon as you have the correct code entered, the button will appear (and you can have some action attached to it).
Don’t forget to turn on auto-refresh for the list. Also, you need to be sure that the codes are unique, otherwise you’ll see more that 1 button :).
Downside of this solution - it could be slow sometimes.
Somewhere around 3:00 on this video Adalo tutorials: multistep customised password reset. ATTENTION - IMPORTANT UPDATES in description. - YouTube I have similar logic implemented (but for user emails).
(2) You can have a conditional action on Button (happen sometimes), set it when Invites → Count, with a filter Code is equal to Input box value, is greater than 0.
So if the Invite with such Code doesn’t exist in the DB, action will not happen.
Best,
Victor.