I encourage clients to do this carefully. I've seen review sites that filter reviews that all come at one time- so I always recommend asking over time rather than in one fell swoop. And, if possible, to ask clients who already have reviews posted as this also increases the likelihood that the review will stick. As far as the ask itself, an email request with the link directly to the business page on the review site (or on their website review capture page) works well and makes it super easy for the client. I've also had some clients do it via a Google form when they're planning on using the testimonials directly on their own website.