Add boxed text to text pallette

If you have inserted a boxed text in the score, as Inspector you should be able to set it as Style (if I understand the question though)

It's not clear what kind of boxed text you mean - something that looks like a normal rehearsal mark, but is used to mean something different? If so, then you can add such an element to your score as staff text or fingering or chord symbol or whatever it is most closely related to, set the properties the way you like in the Inspector, then Ctrl+Shift+drag it to the palette. No particular need to create an actual Style for it. You can use one of the user styles, but the styles are per-score, so if it's something you'd expect to use in multiple scores, better not use the Style and instead simply use the properties.