Every legitimate businessperson knows that risk has to be shared. Therefore, none who post an ad demanding you take a test or create a customized sample without payment is legitimate. You are investing your time. They are investing nothing. Pass on answering that ad.
Are there exceptions to this? Yes. But likely those help-wanted will come from an excecutive search firm. The client offering the big assignment might want two pages of a speech the head of logistics might be giving at an annual meeting of Supply Chain Managers.