Advocacy is somewhat like loyalty: if you have to ask for it, then you probably won’t get it
– and you might even create the opposite effect.
The way to generate trust is to give it, and the way to generate advocacy is to advocate. That could be tricky if, for example, you sell banking software to competing banks: you can hardly be an advocate for them all.
The next-best-things to advocacy are probably referrals and testimonials, because you can certainly ask for those. If the answer is positive, you may incidentally create advocates. If the answer is negative, you have an opportunity to devote some TLC to those accounts.