<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:strip-space elements="*"/>
<xsl:template match="@*|node()">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="/soapenv:Envelope">
<xsl:copy>
<soapenv:Header>
<UserName xmlns="apinizer.com">Test123</UserName>
</soapenv:Header>
<xsl:apply-templates/>
</xsl:copy>
</xsl:template>
<xsl:template match="soapenv:Header"/>
</xsl:stylesheet>
XML