Many Japanese companies require an alternative name entered in half width kana to alleviate this exact problem. Unfortunately, most Japanese websites have a million other UX problems that overshadow this clever solution to the problem.

This is a problem specific to languages using Chinese characters where most only know some characters and therefore might not be able to read a specific one. Furigana (which is ultimately what you're providing in a separate field here) is often used as a phonetic reading aid, but still requires you to know Japanese to read and pronounce it correctly.

The only generic solution I can think of would be IPA notation, but it would be entirely unreasonable to expect someone to know the IPA for their name, just as it would be unreasonable to expect a random third party to know how to read IPA and replicate the sounds it described.