Copying the link did not work for me as suggested in another comment (Nix OS) but I got it to work by adding “network.protocol-handler.expose.com.todoist” = false as a boolean in Firefox’s about:config which prompts it to ask which application to use when clicking the “Open Desktop App” button. I chose the todoist executable and the next time I tried the login process from the beginning it worked. Hope this helps someone.