On Poshmark, listing something with the Boutique tag means that the listing was purchased wholesale or directly from a distributor and is being sold to the public for the first time. To tag items in your closet as Boutique, your account will need to go through Boutique Certification, a short questionnaire - details can be found HERE.
Please note that the majority of shoppers browse listings by applying the NWT filter, not the Boutique filter - so adding the tag shouldn't lead to a significant change in exposure. Boutique items will show up when shoppers apply the NWT filter as well.