What is an R function for filling NA with specific string values?

I am working with the IMDB dataset and am trying to find the best solution for filling in my empty values, for example a show called "Money Heist" has its Genre listed as "Action,Crime,Mystery" however half of the time it will be NA what is the best solution to only changing the "genre" tag for "Money Heist" without changing the thousands of other shows? ive tried df_test <- df %>% if(df$primaryTitle == "Money Heist) {df$genres == "Action,Crime,Mystery"} which failed. I dont need it to be an "If statement" any solution outside of manually changing hundreds of cells would be awesome.