// popup.js
const [tab] = await browser.tabs.query({
  active: true,
  currentWindow: true,
});
if (!tab?.id) return;
Send a one-off message
Once you have the tab.id, you can call:

const response = await browser.tabs.sendMessage(tab.id, {
  type: 'POPUP_TO_CONTENT',
  payload: { foo: 123 }
});
console.log('Content script replied:', response);

// content script
browser.runtime.onMessage.addListener((msg, sender) => {
  if (msg.type === 'POPUP_TO_CONTENT') {
    console.log('Popup says:', msg.payload);
    // …do stuff…
    return { ok: true };    // synchronous reply
  }
});

// popup
const response2 = await sendMessage('testMessage', { message: 'you are a good man' })

// content script
    onMessage('testMessage', (msg) => {
      console.log('testMessage', msg)
      return 'hello'
    })