The easiest way I've found to use the forum 'link' command is to initially in your diatribe just type the text you wish to be the link and a little past it to ensure the link can easily be selected, then select the words to be the link, click on [link] (which puts in both the open and close tags) and enter the URL. I've usually previously copied the URL so the sequence goes [link] < paste > [OK]. The reason for entering text to just past the link is that if the link text is at the end of the line when you select it, your entry point could end up inside the link and require extra effort to sort out. You can also use e.g., [i] and/or [b] on the link text, easiest done before entering the link.
typo edit and for clarification