How soon can I use Anti Dandruff shampoo after a FUT/Strip method surgery

Unless you normally have a problem with dandruff, the itching may not be due to dandruff.  If your doctor did tell you NOT to use anti-dandruff shampoo, then you should listen to your doctor's advice.  You need an examination to find out what is causing the itching.

Hi there, and thanks for your
question. The itching you describe may not actually be related to dandruff.
Many FUT patients report an itchy scalp as a result of the incision healing
process and the growth and shedding of the new follicles.

You are wise to follow your
doctor's instructions. Once you do get in touch with him or her, your doctor
may suggest some topical treatments to help safely alleviate your itchy scalp.
Best of luck.

The most important thing for any patient is to follow the advice of the surgeon he or she chose for the procedure. Your surgeon is responsible for your outcome and if 6 weeks was recommended, you should stick with 6 weeks.The itch experienced post op is not from "dandruff" (which is caused by a yeast!) so there is usually little role for anti-dandruff shampoos (unless dandruff was a major problem pre-op). The itch is usually from healing and shedding follicles. You might consider more frequent washing and use of baby oil to help soothe the scalp.

I generally recommend Head and Shoulders as the shampoo of choice. Use a good conditioner as well to moisten the hair and scalp. 

You need to see your hair loss specialist for a follow up care to find the cause for itching.  It is normal to have itching for a few weeks after a hair transplant but if it continues you should get advise from your doctor.