First appearance: Act 1

Usagi gets this lipstick from Luna at the same time as the necklace that transforms into the Heart Moon Brooch. After she shouts "Moon Prism Power!", she puts the lipstick on and shouts "Make Up!", which makes her transformation continue.

Quite why she needs lipstick to do this is unclear... you'd expect the makeup to go on magically like it does for the other Guardians, so it doesn't seem to serve much purpose other than marketing purposes. However, it does make for a pretty cool toy, since the lipstick comes packaged with the Heart Moon Brooch, and judging by the commercial, the two interact just like in the show. (Successful transformation not guaranteed!)

Anime/Manga Comparison:
This has no basis at all in the anime or manga.