Not sure what you mean by "Insert that code."
If both columns are just text, then this should be easy, but you're going to have to provide the selection criteria.
If A is text, and B is text that you want to insert into the middle of A, somewhere (at like the Nth character) then this formula will do it:
=CONCATENATE(LEFT(A1,C1),B1,RIGHT(A1,LEN(A1)-C1))
where C1 contains an integer N, which is the character at which B1 will be inserted into A1.