What’s the correct hyphenation in “trying to be a decision maker”?

Which of these three ways of writing it is right:

1. decision maker (a space separates the two pieces)
2. decision-maker (a hyphen separates the two pieces)
3. decisionmaker (nothing separates the two pieces)

I’ve looked online and in dictionaries, grammar books, source documents, and multiple documents within my organization, but I still cannot find an answer.

1. Use whatever style guide is appropriate for your organization or audience.

2. If there is no appropriate style guide, then here is a general guideline (and it is only a general guideline):

   • Use a hyphen if the term is used as an adjective: XYZ is a decision-maker tool.

   • Do not use a hyphen if the term is used as a noun phrase: She is a decision maker.

3. Beyond that, @FumbleFingers's comment applies: it's pretty much up to you.