Japanese dynamic characters

I've been looking in the community but didn't find anything.
When using a Japanese font from Adobe Fonts that use dynamic subsetting, it's scanning the website at the beginning and downloading only the needed characters.
What about input fields? Is there any API to call every time the users type a new character in the input field? Or from content dynamically loaded?
If not, Japanese web fonts are kind of useless. Any timeline to implement this kind of feature?