How do I make this large mold for concrete countertop?
Yellow line is where you need to go, make the form slightly longer on the meeting edges so the ends can be recut to a clean joint. the reason being, moisture loss in concrete will change the look of the slab were it looses it rapidly. This means at any joint in the form where water can leak out will force the concrete to cure differently. Caulking the corners will help fix the problem an will also radius the edges a little or more if you choose, when done in the right spots. This should be done on the meeting joint, but of course you don't want a radius where they meet, hence the recut. Have some 1/4" stainless steel rod to drop into the wet mix at the sink front to strengthen those ends. If you put any on the back edge, be sure you steer clear of the area where the faucet is drilled.

You may be aware of this already, but I need to mention this. Concrete slab tops are honed/polished like natural stone is. A stain can be added to the surface prior to honing to add a variation in color too. Melamine will give a good start, but the honing and polishing will give a fine finish.