Here is how I, personally, proceed.
As long time independant new music digger and playlists curator, I have my own website and people who wants contact me, most of time, use this way.
To be honest, I receive daily many requests to listen new music. As I have made this choice, the only streaming service I use is Spotiy. So, I pay more attention to people already having music available on Spotify. However, as I want keep my independance, it's not because I am requested that a song will be added in one of my playlists. It has to fit my taste. I always keep the last word and I am not "to buy" :) but as I dig, daily and deeply the new releases on Spotify, if you have a good recording, I will find it.
I know other independant playlisters who can be reached also through a website. Usually it's mentioned in the description of their playlists.
There is also some playlisters who post in the community. Then can be PMed in the community.
In the case of "official" playlists makers as Spotify, Topsify, Digster, Filtr etc... they have their own curators and their own way to curate their playlists.