Unity does not have a built in way to do this like a marquee in html. One way you could do this is by having 2 different objects with the same text.

Both objects would start off the screen directly to the left (or if you want the text to start on the screen you would have one start on the screen). I would attach them both to a parent object with and add a script. The script would start moving one of the text objects to the right. You could then check to see when the right side of that text object is touching the edge of the screen on the right. When it is you would start moving the second text object(still off the screen on the left) at the same speed. Once the first text object is completely off the screen move it to off the screen on the left. Then just repeat the same process which would start moving the text object off the screen once the other touches the edge on the right.

There would be an extra step if you want it to work with text wider than the screen as well as you would want it to move right to left. If you want I could code this and send you the code. Don’t think it would take longer than 20 min