The string type in FunC is represented by slice, so you need to create new cell and put substrings in it:

	slice charA = "A"; ;; char A
	slice charB = "B"; ;; char b

	;; create cell with "ABBA" string in it
	cell cellABBA =
		begin_cell()
			.store_slice(charA)
			.store_slice(charB)
			.store_slice(charB)
			.store_slice(charA)
		.end_cell();

	;; convert cell to slice (or simply use the cell itself)
	slice stringABBA = cellABBA.begin_parse();

	~strdump(stringABBA);	;; #DEBUG#: ABBA