I am not sure this is possible. When setting up email notifications the password for my email server is shown in plain sight. Not as much a problem on my own computer but I do not want this password visible on other family member’s computers that I am managing.
I agree it should be hidden. There is an issue tracking it here Mail password in advanced properties shouldn't be visible · Issue #3027 · duplicati/duplicati · GitHub
And a greneral issue tracking hiding all passwords from the frontend here Fix not revealing stored passwords from the UI · Issue #2024 · duplicati/duplicati · GitHub
BTW, better if you create application password for D2.