Adding text with Shotcut probably confuses just about everyone the first time they try it.
Once you’ve added a Text filter to a clip, Text can be edited in the “Text” field of the Text filter in the Filters tab. If you’re working on a small monitor, you might need to scroll down in the Filters tab to see it.
(there’s a screenshot in this thread Texts Missing in Exported Video - #5 by paul_streicher that shows the text area really well.)
If you haven’t added a Text filter yet (possibly because you can’t find the filter) see this post