What's the most preferred spelling of auto fill, auto-fill, and autofill?
There is no grammatical superiority or inferiority to open compounds (auto fill), hyphenated compounds (auto-fill) or closed compounds (autofill).
There are forms that are particularly common with particularly common compounds, to the extent that they would be considered preferred spellings. But even then it's common to find two or all three in good use.
There are also some compounds of long standing where all three are still often found (egg beater, egg-beater and eggbeater being an example).
There seems to be a general trend over the last few decades toward fewer hyphenated forms, and more closed forms, but not a so strong as to make hyphenated forms "wrong".
In all, there's little to pick between them. If anything would be objectionable to anyone, it would be the fact that they're relative neologisms, which some people dislike, and such people would prefer to just use "automatically filled in". But that's an objection of subjective taste of some people, not a rule of grammar. I would though avoid a neologism in a formal context, unless the formal text was on a related technical topic.
The only real way to pick between open, hyphenated and closed compounds are:
See what other people seem to prefer in the given case. Pay particular attention to those of the same region as yourself and writing in the same register as you intend to.
If you really just like the look of one more than another, and it's not particularly rare, then by all means go for it: It's an acceptable variation of personal style.
There are though two exceptions:
If a closed compound begins to be thought of as a fully independent word, then you should favour it. That said, if that happens you probably won't think of it as a closed compound yourself, so you won't make that mistake.
If a closed compound matches the spelling of a different word, then you should avoid it, to avoid confusion.
"If a closed compound matches the spelling of a different word, then you should avoid it, to avoid confusion." For example:
- re-lease (not "release") - To lease again.
- release - To let go.