Take the sifted besan in a bowl and add the yogurt. Start adding water little by little. Whisk as you go along. Add the rest of the dhokla ingredients except the baking soda and whisk the batter again. You will add the baking soda right before steaming. Let this batter rest for 20 minutes, undisturbed.
Prepare your instant pot for steaming the dhokla. Pour 3 cups of water in the steel insert and press the sauté button (more). Let the water come to a roiling boil.
While you are waiting for the water to boil, grease a cake tin or sturdy pyrex bowl with cooking oil. Choose a cake tin or bowl that will fit inside your instant pot. Alternatively, you can use a 7" springform pan too. The springform pan makes it very easy to release the dhokla once it's done.
Once the water is boiling, sprinkle the baking soda in the dhokla batter you prepared earlier and whisk again. The baking soda should get mixed with the batter very well, but work quickly as you don't want the bubbles to subside. Pour the batter into the greased bowl or springform pan.
Place the bowl of batter on a trivet with handles (the one that came as an accessory with your instant pot) and lower it into the steel insert very carefully.
Close the lid and make sure the steam release valve is on 'venting'. Press the 'steam button.' Set an external timer for 20 minutes as the timer on your instant pot will not work when you select the steaming function with the steam release valve in the venting position.
After the 20 minutes are over, remove the lid and lift out the trivet with the bowl. Let it rest on the countertop for a bit until it is cool enough to handle.
Loosen the sides of the dhokla with a blunt knife and invert it on to a plate. You can place the plate on top of the cake tin or bowl and invert. If you're using a springform pan, just release the clasps. Cut the dhokla into small squares or diamonds.