Find your runelite folder (eg: C:\Users\"your name"\AppData\Local\Runelite\Runelite.exe), copy the whole folder url, make sure you have "RuneLite.exe" at the end.
Go to OSRS in your steam, right click and go to properties. In the Launch Options section paste the URL you copied and put it in quotation marks, then put %command% after it. When you open OSRS through steam it will open runelite.