So to find out thread ID of a Gmail thread, do the following:

  1. Go to Gmail
  2. If you are using Preview Pane, then disable it (select No Split)
  3. Open a Gmail thread (message)
  4. Thread ID the message will be in Google Chrome browser address bar:

Look at the URL in the address bar. It should look something like a long series of characters and numbers as follows: 

where THREAD_ID represents the thread ID for the conversation you’re viewing.
Remember, the thread ID is consistent for all messages in the same thread, so any message within the conversation will link to the same thread ID in the URL.