Find Out Where Your Community Hangs Out
The best way to promote it is where the people that are interested in the subject hang out, such as Reddit, Facebook, Twitter, etc etc. That's the first and easy part.
The 2nd part is to try and get them to support the proposal. Many people hate signing up to new things, so a well written and convincing sale pitch is a necessity.
Make it Sound Easy
Remind them that they can just simply login in with their already existing social media accounts to support the proposal. Tell them how quick, easy and effortless it is and how they will really be helping you if they do. Tell them how important and useful the community is and how they can be a big part of it.
Social Media HashTags
Also, you can try promoting on Twitter using #StackExchange type hashtags, people that see that will often already have a Stack Exchange account and more lily to follow a proposal.
In my opinion, it's a good idea to use something like Facebook groups or a discord Server to get people together first before a proposal. Once you have enough interest and you understand the mechanism, then go for the proposal.
Social Media Groups & Gamification
The groups you create should contain as many as your proposal followers as possible. Then use the groups to keep reminding people to help market it. People always need a push.
Discord is particularly useful, as you can use roles to gamify participation to help gain momentum.
Offering incentives helps to get people interested. Offer a prize for the person or people who get the most referrals. Offer prizes for the best questions. But be careful with this a lot of people will find crafty ways to scam it, but when done correctly, it can be very rewarding.