How To sign a Pre-Contract

Hey I am doing a season with LFC and I wanted to sign this west brom midfielder named Thorne he is 18, english and has potential, he is valued at 2.5 million and has 3 years left on his contract, i offered up to 10 mill for him but they then asked for 28 mill + charlie adam + 50% of my next sale,, the scout report said I could sign him on a pre-contract, but i dont know how to do that and cant find it anywhere else! please help!