How do I add movie certificate ratings?

World: Is this an R rating for the USA, issued by the MPA? If so, usually the MPA sends its ratings to IMDb directly, so you shouldn't need to submit it yourself. Or is this a rating from another country?

Moovex: I just searched for your film title at the MPA ratings website, https://www.filmratings.com/ , and I didn't see it listed.

One can't self-apply an R rating in the U.S. The R rating is a trademark (as are the G, PG, PG-13, and NC-17 ratings) and can only be used when the MPA has officially rated a film as such.

As advised, a U.S. film rating is officially provided by the MPAA only. Your film has not been reviewed or rated by the MPAA so you cannot list an MPAA rating on your title.