When it comes to a company or brand endorsing an artist, there are varying levels of endorsement. These may include an A, B or C grade endorsement or half or full endorsements etc. Some artists even get paid for using and playing certain equipment and brands. For most up and coming musicians however it's a different story. Before you approach any company, make sure you're ready to purchase new gear and be prepared to get knocked back... you rarely get anything for free and companies don’t just hand out endorsements like party invitations, where it's obviously "more the merrier" lol

It's all about giving them more than they can give you and hopefully then building a long term relationship with the company and the people who work for them, where you both can grow and promote their products together. The key is giving them as much exposure as you can. If they give you great deals or discounts on their gear, in return they would like to see plenty of promotion/exposure on your behalf.  When I first made contact with the companies I still even play now, I was playing in a few different bands and was touring and teaching a fair bit. Musicians in popular band are great examples for companies to promote their gear. The more people see you play the better. Exposure can be in areas such as gigs, live performances, online media, tours, teaching etc, or anything to help build your profile.
 
Loving and believing in the products also helps in all the promotion as well. They need to see that you are passionate about playing and using only thier product(s) not just what ever you can get the cheapest/easiest etc. There’s no point playing and promoting something you ain’t in love with lol. If you feel you have a lot to offer a company only then approach those companies you really love and who's gear you currently already use and believe in. Cheers