You can either use the session-wide link, persistent links, or single-use links.
During testing, you can open this link in up to 1 tabs.
https://otreeexperiment-43b45b69a563.herokuapp.com/join/katipede
For a real session, you should use a room.
Below are single-use links, which you can distribute to your participants. Each link has a unique code for the participant.
P1 | https://otreeexperiment-43b45b69a563.herokuapp.com/InitializeParticipant/vczri4br |