You’re going to have a lot more to talk about with Siri this fall. In iOS 10, Apple Inc.’s upcoming iPhone and iPad operating system, you’ll be able to control third-party app functions using the company’s voice-activated assistant for the first time.

Among the first apps to yield to Siri are big names like WhatsApp, LinkedIn, Pinterest, Square Cash and Slack, along with lesser-known ones including Looklive and The Roll.

I’ve...