t.signUrl(url, args)

Add current context data to a URL so that (as long as it is opened by Trello) the page will continue to be able to communicate with Trello via the Power-Up interface.

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// synchronously returns the "signed" URL
var signedURL = t.signUrl("https://google.com", { anArg: 'hey' });

args is an optional object where you can retrieve the values with t.arg once you load this URL and initialize a t.

In general, wherever possible, Trello does this for you automatically. So when you give a URL to t.popup for example, you don't have to sign it yourself first.

Currently there are two places where you will need to use t.signUrl on your own:

attachment-thumbnail when returning an initialize iframe url:

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window.TrelloPowerUp.initialize({
  'attachment-thumbnail': function (t, options) {
    return {
        url: options.url,
        title: parkName,
        image: {
          url: './images/nps.svg',
          logo: true // false if you are using a thumbnail of the content
        },
        openText: 'Open in NPS',
        initialize: {
          type: 'iframe',
          url: t.signUrl(TrelloPowerUp.util.relativeUrl('authorize-link.html'))
        }
      };
  }
});

attachment-sections when returning the content iframe url:

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window.TrelloPowerUp.initialize({
  'attachment-sections': function (t, opts) {
    return [{
      id: 'abc123',
      claimed: opts.entries,
      icon: 'https://example.com/image.svg',
      title: 'My Section',
      content: {
        type: 'iframe',
        url: t.signUrl('./folder.html', { arg: 'why not' })
      }
    }];
  }
});

This function currently assumes the URL provided to it does not already have a hash component.