EXEC CICS GETMAIN will allocate you memory; using the SHARED option will allow that memory to persist beyond task termination. You will need to store the address returned from the GETMAIN somewhere so every task wanting to access the memory can get to it; if the length of the memory acquired is not a constant then you'll need to store the length as well. When your subsequent program starts, assuming that the size is fixed and that GETMAIN-PTR is defined as USAGE POINTER and has the value from the SET option of the GETMAIN, and LS-AREA is the LINKAGE SECTION 01-level variable that you want to reference the GETMAINed memory with, your COBOL statement would be
SET ADDRESS OF LS-AREA TO GETMAIN-PTR
after which you could reference variables in LS-AREA normally.
Your application code will need to be carefully designed and checked since storage violations are quite easy to get with SHARED GETMAIN memory; if the transaction runs 1000 times a day and allocates a megabyte of memory, and the transaction allocates the memory every time instead of using previously allocated memory, by the end of the day your CICS region has 1 GB of memory tied up in your data structure -- which may (or may not) be possible at your site but DEFINITELY will cause your system programmers to be upset!