Currently, the following system phonetic name (in Japanese: furigana) fields are not available in Marketing Forms.

Schema Name:

yomifullname

yomimiddlename

yomilastname

yomifirstname 

The MS doc states that some system fields are not available on the Marketing form, so it seems that system furigana field is the target.

https://learn.microsoft.com/en-us/dynamics365/customer-insights/journeys/real-time-marketing-form-create

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Some system fields may be hidden, as they can't be used in the form. 

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It is possible to use a phonetic name field on a Marketing form by creating custom phonetic name fields in the Contact and Lead tables.

However, this requires more effort on the part of the user.

We think that the feature to use the system's phonetic name fields on Marketing Forms, it will make Dynamics 365 more convenient and efficient tool for users in Kanji regions, including Japan.