The simplest way you can presently get Rocket League on Rocket League Credits Steam is by means of purchasing a key from a 3rd-birthday celebration key reseller. Many of those sites may be ethically questionable (to place it gently) so we won't be linking to any here, however you may without difficulty discover them with any seek engine.
Another workaround you can do is install Rocket League from Epic Games, after which add it to Steam as a non-Steam recreation. This can be executed by way of starting Steam, choosing "Add a Game" from the lower left nook, clicking "Add a Non-Steam Game", after which finding Rocket League inside the report browser. You may not be capable of RPGAH.COM tune playtime or achievements with this technique unluckily, however it allows you maintain your video games centralised to Steam if that is what you want.