Well first off your input sample isn't well formed XML, so there's something wrong with the sample you've given.
However, if you're trying to build a string that looks like that, this transformation will work. I've modified your original input as well to make it work.
Input
<root>
<customer>company_name
<id>2334</id>
<code>xsTu4</code>
<mail>info@comany.com</mail>
</customer>
</root>
XSLT Transformation
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
<xsl:apply-templates select="root"/>
</xsl:template>
<xsl:template match="root">
<xsl:element name="links">
<xsl:apply-templates select="customer"/>
</xsl:element>
</xsl:template>
<xsl:template match="customer">
<xsl:element name="page">
<a>
<xsl:attribute name="href">
<xsl:value-of select="concat('detail.aspx?id=',id)"/>
</xsl:attribute>
</a>
</xsl:element>
</xsl:template>
</xsl:stylesheet>
This produces an output of the following.
<links>
<page>
<a href="detail.aspx?id=2334"/>
</page>
</links>
Without more details, I can't really help much more.