How to create a new column and add a random identifier to it with miller

I want to add a column with a randomly created "case number" to my csv file. The first 2 letters of the casenumber must be any letter from A-Z in capitals. followed by 5 random numbers. input:

COMPANY,NAME,STREET,ZIP,CITY,IBAN
Test Ltd,John,Big Ben 343,4343,London,UK2348020384
Test Ltd,Kate,Big Ben 343,4343,London,UK4389223892
Test Ltd,Jake,Big Ben 343,4343,London,UK3892898999

output

COMPANY,NAME,STREET,ZIP,CITY,IBAN,CASENUMBER
Test Ltd,John,Big Ben 343,4343,London,UK2348020384,IN84903
Test Ltd,Kate,Big Ben 343,4343,London,UK4389223892,TY93842
Test Ltd,Jake,Big Ben 343,4343,London,UK3892898999,OL34307

How to do this with miller? I have the following command ready

mlr -I --csv put '${CASENUMBER}=xxx' then \
	reorder -f COMPANY,NAME,STREET,ZIP,CITY,IBAN,CASENUMBER input/input.csv

What to add to the above command exactly?