how to test Onesignal push notifications on localhost?

It should be https://localhost not http://localhost. OneSignal treats localhost as secure connections.

Besides configuring OneSignal via dashboard, your OneSignal.init in your browser call must have the allowLocalhostAsSecureOrigin option:

OneSignal.init({
  allowLocalhostAsSecureOrigin: true,
  ...
})

More at https://documentation.onesignal.com/docs/web-push-setup-faq#section-how-do-i-test-my-site-on-a-local-environment

read in this doc https://documentation.onesignal.com/docs/web-push-setup-faq#section-how-do-i-test-my-site-on-a-local-environment- and localhost must run in port 8000

You may use Postman. Just follow this steps, found at: One Signal documentation

• Select POST and use url https://onesignal.com/api/v1/notifications
• Set headers, like this:
• And, finally, add the push body

Surprisingly, simple. Enjoy.

For anyone else struggling with this, the answer is to add 'http://localhost' but when the configurator comes back with a message similar to 'localhost is already taken. Please enter a different name.' jut scroll down and type in a different subdomain. It's not very intuitive but that worked for me.

In order to test onesignal push notifications over localhost, you should enable https://localhost on your test environment. The following great post help me to setup it on macos, but I think you could implement it in other operative systems:

https://medium.com/@jonsamp/how-to-set-up-https-on-localhost-for-macos-b597bcf935ee

When you are able to start your web server at https://localhost:8080 , you should configure on OneSignal App the following:

Site Url :  https://localhost:8080
Check "My site not fully HTTPS"
CHOOSE A LABEL :  your-test-env.OS.TC

By completing the setup suggested by Onesignal documentation, you will be able to test push notifications on your test environment.