Lena is obsessed with Kara's scent, from day one. Kara doesn't really use anything extra ordinary, its just she is Kryptonian. That's it. But Lena doesn't know. From the moment Lena met her to finding out Kara is Supergirl. Lena was on a journey to find out, what exactly Kara is using, she even straight up asked her but Kara just smiled and told her its a solid perfume from a small business and she can get it for Lena. Lena actually starts learning about perfumes because of this and made some herself, however its always a fail. So the only thing left to do is hold Kara as close as she can whenever she gets the chance and try to not let Kara know Lena is smelling her like an obsessed Alpha (its a fail, Kara just doesn't mind as long as Lena is holding her close).
My girl was having withdrawals because of this in the divorce era and when everything is settled down, she tells Kara that she is going to find a way to bottle Kara's scent. Which is weird to Kara cause nobody ever really commented on it but she just smiles thinking maybe it was cause nobody has ever been close enough before Lena.