External partners are often forgotten at project completions, or treated as an add-on. That's a missed opportunity. If a supplier or freelancer has made the difference, they deserve recognition just as much as an internal colleague.
Ways to involve external partners: invite them to the joint project completion (if the relationship allows). A separate thank-you dinner or drinks specifically for external parties. A personal, handwritten thank-you note with a concrete mention of their contribution. A reference, recommendation or LinkedIn post that makes their work visible.
The signal you send by thanking external partners: we are reliable, we value collaboration, and we want to do this more often. That's the foundation of a long-term business relationship.