Power Query: How to add a calculated row?

HI, Power BI professionals~

I have +300,000 pieces of order information. 
I am analyzing the savings compared to the unit amount of the previous month.

1. The left side is my data. Right side is the data want to create.

2. All data is generated additionally by the [EndPeriod]. => Flag : Add 
 - ex) the data of [Category = AAA & Index  = 1] will only be checked for additional orders until [EndPeriod] = 202304.

3. [Dif.Amt] is the difference from the [AMT] in the previous index.

I'm waiting for your help.