How can I make link action

Jira Development Jira Cloud
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2

Hello!
I don’t know if it fully answers your question but this snippet should allow opening new tab to external page:

var flag = AP.flag.create({
  title: 'Successfully created a flag.',
  body: 'This is a flag.',
  type: 'success',
  actions: {
    'actionkey': 'Click me'
  }
});


AP.events.on('flag.action', function(event){
    if (event.actionIdentifier === ("actionkey")) {
    	window.open('https://www.atlassian.com', '_blank');
    }
});

if you need to open the page relative to your Jira url then you could use this API: Navigator
Also there is some discussion about similar topic here: Show AP.flag with dynamic URL in action
Feel free to contact me if you have more questions.
Cheers!