I see the «Create your Magento API Credentials» instruction:
xsellco.com/support/magento-api-key-username-password
But it is for Magento 1, not Magento 2.
Magento 2 backend does not have the «System > Web Services > SOAP/XML RPC – Roles» and «System > Web Services > SOAP/XML RPC – Users» menu items.
Moreover, the Magento 2’s SOAP authenticatio flow is completely different from Magento 1. Magento 2 does not provide a password authentication, it provides an OAuth access token authentication instead: devdocs.magento.com/guides/v2.0/get-started/soap/soap-web-api-calls.html#auth
So, how to integrate Xsellco Helpdesk with Magento 2?
I have asked it from Xsellco: support@xsellco.com