Using New Outlook on Windows for personal email, calendar, and contact management
Microsoft added “Personal dictionary” to New Outlook’s Editor settings, but Microsoft’s own release notes say these features are rolled out gradually. This explains why some accounts/builds have the option while others do not: https://learn.microsoft.com/en-us/officeupdates/release-notes-outlook-new
Since your New Outlook identifies “Rownhams” as a spelling error but provides neither “Add to dictionary” nor “Personal dictionary”, there is unfortunately no reliable setting you can change locally.
The simplest workaround is to use classic Outlook and add the word there, or ignore the underline until Microsoft enables/fixes Personal Dictionary for your New Outlook account.
Microsoft’s dictionary instructions: https://support.microsoft.com/en-us/word/add-or-edit-words-in-a-spell-check-dictionary
I would also send this through Help > Feedback in New Outlook, because your screenshots clearly demonstrate the missing feature.