This indicates is not really possible:
https://learn.microsoft.com/en-us/answers/questions/460124/customize-text-on-exchange-2016-owa-change-passwor-1.html
How to change exchange OWA password notifications
Hi
The minimum password length of my AD policy is set to 10 characters, but when OWA changes the password prompt,it prompts at least 8 characters,as shown in the figure below.Can this notification be changed?
Exchange Server Management
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Andy David - MVP 157K Reputation points MVP Volunteer Moderator
2022-12-15T12:42:07.563+00:00
1 additional answer
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Yuki Sun-MSFT 41,376 Reputation points Moderator
2022-12-16T06:50:03.277+00:00 Hi @Tony Mu ,
Agree with Andy that I am afraid it's not feasible.
Although per my research, there used to be a non-official trick for this requirement in certain outdated builds of Exchange (see this link), based on further research (See discussion here)and a quick test in my Exchange 2019 lab, seems to me that it doesn't work anymore.
With the above being said, I've just tried submitting this via the dedicated feedback portal for Exchange server. I'll leave the link below so that you or other community members can help add vote or comment there. Hopefully this feature request can be noticed and added in the future.
Customize the suggestion text when changing password in OWA
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