If you're not really interested in drawing a line or prefer using a real vector drawing program like Inkscape instead of Scribus for this, you might also consider this tip that many people ask for: make a text follow a line. There are no tools for this in Scribus so don't look for it in the toolbar. Of course, it is still possible and here are some steps to follow:
First of all draw a line (with the Insert Freehand Line tool, for example) or a shape.
Then add a Text Frame, write your text inside, and set it nicely if you already know what you want.
Select them both.
Go to Item | Attach Text to Path.
You can see that the line is now hidden, but it is still there. If you then want to separate the text and the line, just go to Item | Detach Text from Path. By selecting the line and the text at the same time, Scribus understands what needs to be joined. Just take care of selecting only one line.
If you want to modify the text, double-clicking...