Is there a way to get focus on InputBox?
Topic on Extension talk:InputBox
I don't think there is (certainly not documented).
There is a semi-related FAQ at w:en:Wikipedia:FAQ/Main_Page#Why doesn't the cursor appear in the search box, like with Google? which might help explain why that isn't a standard feature. I'm not a developer, but the code in w:en:MediaWiki:Gadget-searchFocus.js might be adaptable to that use, if you absolutely must have that functionality somewhere?
Hope that helps.