It’s easy… Highlight the text you want to block quote FIRST and then click the Blockquote button. It will automatically quote the text you have highlighted. You can also use a keyboard shortcut instead of clicking the buttons. Just mouse over the options in the editor and a tooltip will popup showing you what the key combinations are. Blockquote is Ctrl+Shift+9. Just highlight your text and and press the shortcut keys. Done.
Most of the options in the editor work the same. Simply highlight the text you want to edit and then press the button or the shortcuts.
The forum is run on Discourse forum software. Looks like there is a guide here:
…that covers the various facets of using the forum.
You can also use Markdown for formatting text as well: