New tags can only be created on relevant questions by users with sufficient reputation. However, can sponsored tags be created proactively without an existing question by an interested sponsor?
As others pointed out, tags don't exist (for long) without questions. But a company representative can create a sponsored tag along with a question. After all, anyone who can create a tag can also post a question with a tag. A recent example is actions-on-google which was created by a Google employee who posted a self-answered question about the product.
If I remember correctly, it also happened that an SE employee created a sponsored tag by adding it to some existing questions about the product; presumably this was at the request of a company that lacked a representative with enough reputation. (I think this is what happened with [sonos] tag on SO, which was created by hairboat; the tag is not sponsored now but that may be because the sponsorship expired.)
I doubt this is true, primarily because what rene already commented:
If that would be possible and the system works as I'm told such tag would only live till 03:00 UTC when tags without questions are removed.
That is what we know about how the system works. Going back: I doubt you will ever need having a tag first and then the question. From what I have experienced, there are always first questions and when a company sees future in sponsoring the tag, it will get sponsored. I don't think sponsors will have any interest in starting on SO without a minimal basis.