How I can Add request via API on HR Instance?

How I can Add request via API on HR Instance?

I have 2 instances IT & HR
How can I Add request via API on IT Instance?
I cannot pass parameters to this URLs:
http://itsm/api/HR/v3/requests?
OR
http://itsm/api/hr/v3/requests?
OR
http://itsm/api/portal/hr/v3/requests?
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Error: Invalid URL 

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