I usually take a request for a work sample to mean “an example of something you created” rather than “an example of work you did for a past employer.” The latter could serve as the former—assuming you’re allowed to show it, which it sounds like you’re not in this case—but it’s not the only way to go about it.
You could make a sample or mockup of something similar to what you’ve made previously without including proprietary information. If you need branding guidelines or a set of content to work from to get started (maybe you’re building a website or something), you could search online for publicly available versions of those documents from other companies to demonstrate that you’re able to follow guidelines when requested.
I swapped from Sync to Summit and have been loving it. The dev’s super responsive too, which is a big plus.