Testimonials alone aren't a good enough marketing tool, and certainly an anonymous testimonial won't be good enough then. There's no secret to marketing, just talk to your users and potential users and ask them what they want to see to commit to using your product, and if they say they want to see testimonials, then you know to focus on testimonials. Although, if you can demonstrate that you are solving their problems, and you are at this early enough stage that you can spend time on each customer, you likely won't need testimonials, and of course if you don't have any then you need to focus on getting customers that you can get without needing testimonials. But there definitely isn't some sort of ruleset like "if you get two testimonials then you unlock 3 million ARR" - you understand your customers better than anyone and need to play to their specific needs.
In terms of how to go about getting testimonials, ask your customers if they are willing to write testimonials or reviews with their name on it or allow you to use their name on your website, and if they say yes and write a good one, put it on your website. The mechanism genuinely isn't more complicated than that, you just need consent from everyone.
Good luck and congrats on having a big brand as your client!