It should compute automatically if your client is eligible.  There are, however, specific input that you should consider for F.8880.  You'd find these under Credits > CTC, EIC, Oth Credits, on the "Retirement" tab.

When you're not clear where to make an input, you can always use the search box on the left by typing in keywords.  In this case, if you type in 8880, the input screen mentioned above would be the first search result.