Setup your prompts in the OneSignal WordPress Plugin, so that users can subscribe to push notifications. You can do this in the Configure section of the plugin.

Subscription Bell

A small icon at the corner of your page that users can interact with to subscribe

Slide Prompt

A small modal that comes from the center of the page and asks if users are interested in signing up.

HTTP Pop-Up (HTTP-only) - a pop-up window that appears after other prompts that is required for HTTP sites.