I saw that I can create a task for a contact through the API. 

Is it also possible to create a task for an Opportunity?

If so, what should I change in the below?

https://crm.zoho.com/crm/private/xml/Tasks/insertRecords?authtoken=Auth Token&scope=crmapi
&newFormat=1&
xmlData=

<Tasks>
<row no="1">
<FL val="Subject">Call</FL>
<FL val="Due Date">2009-04-20</FL>
<FL val="Status">Deferred</FL>
<FL val="CONTACTID">2000000017017</FL>
</row>
</Tasks>

Thanks