How to generate number of submit form in sequence?

Hi,
I making submit of form in sequence order. Currently I am making it randomly. But I was thinking if there is anyway I can do in sequence order.
For example when user submit form 1st time it says D001, then again he submit the form it's create D002 and so on.
I was using this which create random  result.
Random().Next(1, 100)
I have tried for loop but it just creat D001 and doesn't move on to D002 always create D001 so can you please help me out.

Thanks