I think that it can be inserted just before the last “}” that closes the RTF. The sequence to be inserted is:
\viewbksp1 {\*\background {\shp{\*\shpinst\shpleft0\shptop0\shpright0\shpbottom0\shpfhdr0\shpbxmargin\shpbxignore\shpbymargin\shpbyignore\shpwr0\shpwrk0\shpfblwtxt1\shpz0\shplid1025{\sp{\sn shapeType}{\sv 1}}{\sp{\sn fFlipH}{\sv 0}}{\sp{\sn fFlipV}{\sv 0}}{\sp{\sn fillColor}{\sv 3506515}}{\sp{\sn fFilled}{\sv 1}}{\sp{\sn lineWidth}{\sv 0}}{\sp{\sn fLine}{\sv 0}}{\sp{\sn bWMode}{\sv 9}}{\sp{\sn fBackground}{\sv 1}}{\sp{\sn fLayoutInCell}{\sv 1}}}}}
Maybe it can be simplified.
The background seems to be recognized by the RichTextBox control, Word and WordPad editors.