Hello @Angelfire,
Thank you for the additional information. I don't have issues filling on Windows. That said, please let me know the popup shows up when you use OneNote's desktop app or OneNote website on your browser.
If you Sign in to OneNote on a browser, I confirm that Safari indeed does not suggest or show the inline menu. You can try another browser on your Mac such as Edge, Chrome or Firefox to fill in OneNote. If you want to keep using Safari on your Mac, you can try these steps to create a OneNote item and let me know how it works.
Open the 1Password desktop app > Select the plus icon in the menu bar > Enter your MS account's email and password > Paste this link to the Website: https://www.onenote.com/hrd?wdorigin=ondcauth2&wdorigin=ondcnotebooks > Save. When you need to sign in to OneNote, you can select the 1Password icon in Safari and select Autofill on the OneNote login item we just create there.
Best regards,
Nhat Nguyen.
https://support.1password.com/