The URL looks like this;
https://maker.ifttt.com/trigger/amex_monzo/with/key/key (highlighted text is a button rather than text).
Also key definitely matches. I need to quadruple check your instructions.
Edit: I started again from the ground up. Disconnected Monzo from IFTTT and deleted all webhooks. Also removed Siri shortcut to start again and have followed instructions as meticulously as I can, and it still doesn’t work.
When I initiate the Siri shortcut, it’ll say the money has been transferred, but I open Monzo and there’s no transfer initiated.