The Safari 12.1 Release Notes states that web push permission prompts now require a user gesture in order to display. This is a hard restriction of the browser and cannot be bypassed.
This means that you are not able able to prompt users for opt-in on page load, the user will have to perform an action that can be used for triggering the prompt functionality of the Xtremepush SDK. Implementing a value exchange prior to triggering the browser prompt will resolve this.
Please refer to our article Web Push Permissions Prompt Settings for more information on custom prompting.