Add custom message to PO_DATA for PO_STANDARD_XSLFO.xsl

How do Add a custom message to PO_DATA for use withing PO_STANDAR_XSLFO.xsl stylesheet?

<fo:table>

<fo:table-column column-width="180mm"/>

<fo:table-body>

<fo:table-row>

<fo:table-cell>

<fo:block xsl:use-attribute-sets="cover_page_text" >

<xsl:choose>

<xsl:when test="PO_DATA/REVISION_NUM > 0 and PO_DATA/AMENDMENT_PROFILE ='Y' and PO_DATA/SIGNED='T' ">

<xsl:value-of select="PO_DATA/AMMENDMENT_MESSAGE"/>

</xsl:when>

<xsl:otherwise >

<xsl:value-of select="PO_DATA/COVER_MESSAGE"/>

</xsl:otherwise>

</xsl:choose>

</fo:block> </fo:table-cell>

</fo:table-row>

</fo:table-body>

</fo:table>

I would like to change COVER_MESSAGE  to CUSTOM_COVER_MESSAGE.

I have already built custom_cover_message in the app using application_developer/message forms and generated it.