Tonic, subdominant and Dominant markers

Not sure I understand what you are looking for. I would not use the tuplet bracker for this but use the last line in the palette, once in the score, right click on it , line properties and add hook at the start and the end. Then add the text you want with Ctrl + T.

The thing to realize is that there is no such thing as a tonic, subdominant, or dominant marker. That is, those aren't special musical symbols with specific meanings. They are just something the author of that particular book cooked up to illustrate a particular musical concept. Those are just ordinary text markings that also involve brackets. And yes, the Lines palette is your best bet for doing that in MuseScore. But you normally wouldn't be creating something like that in real music - it was for textbook illustration purposes only. Which is to say, those are markings textbook writers might use, not composers. But again, nothing special about the word tonic, subdominant, or dominant in this context - they are just words placed in brackets. I mean, those words mean what they mean, but there is nothing special about using a the word with a bracket around it to mean something. He might just as well use the same type of notation to label the "treble clef" or a "measure", but those words don't normally appear in actual music either - justt textbooks.

Tonic, Subdominant and Dominant are just a set of names for the 1st, 4th and 5th degrees of the scale, which are often used in teaching the theory of chords as you can use the triads founded on those 3 notes to harmonise any piece of music.

So in the key of C the Tonic will be C, Subdominant F and Dominant G

In the key of Eb Tonic=E♭ Subdominant=A♭ Dominant=B♭

So in this theory book, the writer is trying to get you to add these for different keys to help you to understand the concept.

You will find other names used for degrees of the scale too as you progress in your music knowledge.

HTH
Michael