I’m writing an email to ask current clients to participate in a usability interview. Here are some resources to help word the asking:
(page 58) https://media.nngroup.com/media/reports/free/How_To_Recruit_Participants_for_Usability_Studies.pdf
and just because I like it, this: https://www.interaction-design.org/literature/article/how-to-recruit-users-for-usability-studies
And later I received a little help from my boss, in a nutshell:
One of our initiatives is to learn from our customers how they use our products. During the usability discussion, we will do a deep dive into your usage patterns to get an idea of how you use our product. Our goal is to uncover any bottlenecks, to understand any pain points, and ultimately to improve the customer experience.