Use the below formula to show the previous 12 months

 

Numbervar Yr :=ToNumber(Left({?My Parameter},4));
NumberVar Mn :=ToNumber(Right({?My Parameter},2));
Numbervar i:=0;
Datevar dt:= Cdate(Yr,Mn,1);
StringVar tt:="" ;
for i :=1 to 12 do
tt:= tt+ Totext(Year(dateadd("m",-i, dt)),0)&Totext(Month((dateadd("m",-i, dt))),0)+CHR(32);
tt;

 

 

P.S: Replace the {?My Parameter} field with your YearMonth input field.

The above formula will shows all the previous YearMonth values in a single Text box,

if you want to treat those values as separete then you need to load those values to an Array based on Space(chr(32)) value.