If you have not started a new post:
Select the text with the mouse, a button with just the word Quote will appear, then click it and a new post with the quote inserted will be created.
If you have started a new post:
Move the cursor to where you want to insert the quote, then select the text with the mouse, a button with just the word Quote will appear, then click it and the quote will be inserted.
Just type ``` on one line, enter a blank line, then enter ``` on the third line by itself.
Now insert or type the code in place of the blank line.
Also see: Language highlighting
There is https://try.discourse.org/ where you can play all you need.
You could also use @EricGT and request that I reformat the post.
There is also the Raw Post button at the bottom of each post that will show you the post as how the user entered it and includes all of the formatting as text.
To use the Raw Post button, click on 
then 
.
Also see the other topics in this category: Discourse Forum Feedback