Psoriasis on hand. Can I soak hand in coal tar shampoo such as tgel...If yes how long should it soak?

You can, but... It won't hurt to try, but don't expect significant improvement unless your psoriasis is very mild. Tar preparations are (somewhat) useful for psoriasis, but tar shampoos aren't strong enough to thin plaques on hands significantly. Much more effective treatments are available; see a dermatologist.