Either
<a href="{url}">mylink</a>
Or
<a>
<xsl:attribute name="href">
<xsl:value-of select="url"/>
</xsl:attribute>
mylink
</a>
The former is an attribute value template. Read about them in more detail here.
Caveat: This works only if your input XML looks similar to the following (i.e. if url
is really an element and attribute
its parent element).
<attribute>
<url>www.google.com</url>
</attribute>
You might have to adapt the above for your XML input (since you do not show it).