How do people get brands identities on their site? (Social Proof)

I had a call with a customer yesterday to talk about some new features. I hadn't met the customer before, and during the call I found out that they work at huge company.

I've been wanting to add some testimonials to my Sign Up Page for a while, so I figured I would pop the question: "Would you be willing to give a short testimonial about the app?"

The customer was pretty hesitant. Although they said they would be willing, they immediately said, "but you can't use THE_COMPANY name on the site". Presumably because at major companies I guess you have to declare every app your using on the job for security reasons.

On the other hand, when I'm browsing indie developer's apps I feel like all the time I see different brands/companies that are using the app.

I guess I'm wondering– how do people approach this? Assuming they know someone at company X uses their product– do they manually email users? Do they email the company? Do they add the brand without asking and wait for an angry email?

How have you approached this in your work and what are your recommendations?

Thanks!