It should be like this:
<member>
<name>custom_fields</name>
<value>
<array>
<data>
<value>
<struct>
<member>
<name>key</name>
<value>
<string>lrp_rating</string>
</value>
</member>
<member>
<name>value</name>
<value>
<string>5</string>
</value>
</member>
</struct>
</value>
<value>
<struct>
<member>
<name>key</name>
<value>
<string>lrp_album</string>
</value>
</member>
<member>
<name>value</name>
<value>
<string>sealed_knot</string>
</value>
</member>
</struct>
</value>
</data>
</array>
</value>
</member>
