You can't attach a behavior to plain text. Tags like p and span don't generate events in browsers, so there's no way to trigger an action from those tags.
However, you can attach a behavior to a link. Therefore, to attach a behavior to text, the easiest approach is to add a null link (that doesn't point to anything) to the text, then attach the behavior to the link. Note that if you do this, your text will appear as a link. You can change the link color and remove the underlining if you really don't want it to look like a link, but site visitors may then be unaware that there's a reason to click that text.
javascript:; in the Link text box. Be sure to include both the colon and the semicolon.
NOTE: You can instead use a number sign (#) in the Link text box if you want. The problem with using a number sign is that when a visitor clicks the link, some browsers may jump to the top of the page. Clicking the JavaScript null link has no effect at all on the page, so the JavaScript approach is generally preferable.