I soak my rapid rooters in a mild nutrient solution before putting seedlings in them. I haven’t always done this, and used to just soak them in plain PH’d water.
I user a cloner as an intermediary step to get my roots longer and more ready for the final DWC system, however, once you have roots coming out of the rapid rooter, you can place it in your system and foliar feed (spray with a spray bottle) the plant a couple times a day until the roots grow long enough to reach the water in your system.
During this time, you can go with no nutes (until the cotyledons are used up - those are the first set of leaves on the plant, the little round ones), and then start it on a 200-300ppm solution. I usually don’t spray them with a nutrient solution, and only plain PH’d water. Once I see a couple nodes of “real” leaves (the saw-toothed types of leaves we all recognize as cannabis leaves) then I start them on a seedling nute formula.