A simple way, assuming Table View, is to add the 'junk' option to the menu and then click it.

How do I add the 'junk' option to the menu - which menu?

And what is Table View?

And what is Table View?

I just click the flame icon in the message list. It will change to a reddish colour when a message is marked as spam and usually the message will then go to the spam folder.

Perhaps read the support document for Spam/Junk https://support.mozilla.org/en-US/kb/thunderbird-and-junk-spam-messages

If the 'quick filter' button is shown above the message list pane, click the icon to the right to select table view (which shows columns, such as subject, from, date, etc) and you can then click the top right icon on message list top line to tick desired columns, one of which is 'spam'). If the 'quick filter' isn't shown, you need to activate 'message list header'. See two attachments