You may check for the hosting account details and access to the Plesk from the client area itself.
Steps:
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Login to your client area
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Proceed to -> Services -> My Services -> Choose the hosting and click on Manage Product and the nameservers details will be shown there.
By default the nameserver would be:
ns184.mschosting.com
ns185.mschosting.com
ns186.mschosting.com
Sample as below: