How can I post message from embed iframe

Hello 🙂
Im working on oauth signin on Figma plugin. (fallow this documentation : link)

Now Im stuck in send token from UI view to figma plugin…

Theres no error but I can’t get message on plugin

//code.js

figma.showUI(
      `<iframe src="http://localhost:3000/oauth"></iframe>`,
    );
  }

...

// oauth 
parent.postMessage(
   {
    type: "some type",
    pluginId: '12341234'
  },
  'https://www.figma.com'
)

How can I send message on iframe embeded website.
or this is wrong way oauth signin??