The return key is not an enter key.
Several websites require the enter key to submit the text, the return key doesn’t produce the function.
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Depending on your particular physical keyboard, try Function-Return, Option-Return, or Command-Return chords, or use the Enter key on an extended keyboard.
On the iPad on-screen keyboard, the return key is what is available. Hopefully there’s some other button or control available on the webpage.
It’s fairly rare for laptops to have separe return and enter keys.
Check with the website maintainers, too.