ShareX is a free and open-source screenshotting and screencasting program, which has a possibly rather overwhelming set of features and settings.
But without going through all the options, it can be a very easy to use tool for just making a simple screenshot and creating a shortcut link that can be used for adding a screenshot to your forum post.
This is a quick and short tutorial to accomplish just that.
Start ShareX, select 'Capture', and then from the underlying menu select your required method of capturing.
For this example you could use 'Region'.
That will allow you to select the area of the screen you want to capture.
When that area has been captured, you will see the filename of that capture appearing in the main panel.
Right-click on that filename, select 'Copy', and then 'Forum (BBCode) linked image'.
That will place a shortcut into your clipboard, which you can directly paste into your forum message.
You can also set ShareX up, so that when you right-click on an existing image file, and then select 'Upload with ShareX', you get presented this panel, from where you can also quickly create a working link.