How do you say rub it in'?
How do you say ..
This product is moisturizing and lasts for days.
Rub it in.
You have to be careful with machine translations they are not always (or even usually) very accurate.
Este producto está hidratando y dura por días
You wouldn't use the gerund here or estar, you would need to use a "ser adjective" in this case to describe it as an inherent characteristic of the product itself.
Este producto es hidratante...
The use of "adentro" here I believe would be incorrect. Also, you have to be careful when using "frótelo" because depending on context it can come of sounding profane. It would probably better to say it using "echar:"
The translation to the sentence is "Este producto está hidratando y dura por días. Frótelo adentro."