To get the message ID, you need to click on its number (#xx) which you can find on top right of the post next to +Merit.
It is one way to find a post ID. Another way is to click on "Re: How to find ID number ?" of that post (I name it as a post title), then you will get the post ID at the browser.
For example, it is your post link
https://bitcointalk.org/index.php?topic=5397719.msg60073131#msg60073131
60073131 is your post ID. From that link, you have a topic ID as well, that is 5397719.
There is another way to get topic ID, click on "How to find ID number" at the top "Bitcoin Forum > Other > Beginners & Help (Moderator: MiningBuddy) > How to find ID number ?", then you will be directed to this one
https://bitcointalk.org/index.php?topic=5397719.0
5397719 is the topic ID. This link directs you to the Opening post of this topic.