An invitation
Open this link on an iPhone - it leads into Tysha.
If the app is already installed, the link opens it by itself and the invitation is right there. If not, install Tysha and open the link again, with the same tap.
The code in the link is one-time, lives fifteen minutes and says nothing about who invited you. It sits in the part of the address a browser never sends to the server - so neither we nor whoever passed the address on can see it.