If you have a two step of subscription currently and you don't want an intermediate window opening, or you don't want people to click on allow twice so you should go for Single step opt-in. 

Advantages of Single Step - 

1) The subscription rate is twice the double step opt-in.

2) You collect subscribers at your own domain. 

You can enable Push Single Step opt-in if you have an HTTPS website and you have installed the service-worker.js file in your root folder. 

This is how Single step opt-in appears - 

To enable Push Single Step, you will have to navigate to Settings > Subscription Dialog Box Settings > HTTPS > Push Single Step opt-in 

Below is the screenshot attached for your reference. 

Please note to use this Single step opt-in you need to have service-worker in your root folder else your subscription will start failing. 

If you have not added service-worker.js file then please navigate to Settings > Installation Settings > HTTPS > Download Service Worker file > Unzip and place it in root folder 

Such that it is accessible from https://www.yoursite.com/service-worker.js location. 

In case you face any issues please let us know.