My first dwc grow when I fill my reservoir do I fully submerge my seedling ?

Noooooo! Water to the bottom of the basket with an air stone. If the rockwool stays wet it will lead to rot pretty quickly.

Cool so seedling will grow and reach for the notes as it needs it?

As long as you have an air stone and pump the bubbles popping will be enough to keep things happily moist in the basket. When the roots start to grow out of the basket let the water level drop naturally.

Important: you may need to top water a bit at the beginning while the roots grow out of the rockwool. And no nutes for the first week, just PH 'd water (5.5-6)

you could germinate the seed and grow the seedling outside of the system, in a cube/plug/cup, then transplants into the system when its ready for fertilizer

As @somebodyhearted said, DWC systems are never to be used for seedlings. Only plants with root systems. Grow it out of the bucket cor the first week or two. Once it has a few roots poking out of the basket, put it into the system. Hand water it and also, don't overwater it. Rock wool holds a lot of water and STAYS WET for a very long time.