First, let’s be clear about this: in an endorsement deal, the artist endorses the product or brand. The product or brand doesn’t endorse the artist. If an artist claims to be “endorsed by” a company, that is incorrect word usage.
An endorsement deal means that an artist agrees to be publicly associated with a product or brand, presumably because the company thinks that will encourage more people to purchase their products. In return, the artist generally receives some kind of compensation, which often takes the shape of free or discounted products. The contract might specify some requirements for the artist to fulfill, such as having their name and image used in advertising, appearing at the company’s publicity events, or plugging products on social media.