First you have to be a regular at shows and have a significant presence in shows and social media, you have to be good and show results. After all, you'll be representing a brand and part of representing a brand is selling and making them look good. If you have a influence in the industry and show them results it will be much easier for you to help them sell and that's what matters to them. Most riders that get sponsors are either pros, have contacts with someone from the brand, have a big impact on social media or are regulars at shows and often win
To be a professional rider it must be your job. A junior rider is under 18. You can not be partnered with any brand of you are an amateur rider
How do you become a professional or junior? Is it the height? Shows you go to?
Be a successful professional or junior rider.
Basically what Tristyn said! I wish there was an easier way!
From what I've seen, basically you either need to be pro, have connections, or have a huge social media presence