You cannot incorporate a SEQ field in a style definition. Sorry.
Instead of a text box, you could use a Frame, which can be a part of a Style definition. I'm not sure how this would work with Automatic numbering in the same Style.
Frames and Textboxes in Microsoft Word
How to create numbered headings or outline numbering in Ribbon Versions of Word by Shauna Kelly
I suspect you would want the Style with the frame to be followed by the numbering style, or
vice versa.