The only manner you can presently get Rocket League on Steam is Rocket League Credits by means of shopping a key from a third-birthday party key reseller. Many of those sites may be ethically questionable (to put it lightly) so we may not be linking to any here, but you can without problems locate them with any search engine.

Another workaround you could do is LOLGA.COM install Rocket League from Epic Games, and then add it to Steam as a non-Steam game. This may be finished through establishing Steam, deciding on "Add a Game" from the decrease left corner, clicking "Add a Non-Steam Game", after which finding Rocket League in the report browser. You won't be capable of track playtime or achievements with this approach unfortunately, but it enables you preserve your video games centralised to Steam if that's what you need.