Bring the Hindi text into the TeXPage

Select (using mouse drag with left mouse button pressed, or using Ctrl + a to select all, or using right click).

Click on the TeXPage tab to get back the mathematical expression preview page and overwrite (paste copied content over) the existing edit area content with the selected and copied (into memory) Hindi (+ mathematical expression in TeX/LaTeX syntax) content (using Ctrl + c, or right click).

Sample snapshot: