This works with clubs that already have players on loan, but you can also loan in your own player and buy him that way. I used Arsenal, purchased Ceballos outright and it popped for example.
When you do find the player you want to buy, navigate to the squad hub, go to the player, and click “approach to buy.” After that you proceed through the motions like you would a normal transfer. Should pop up shortly after the player signs his contract.