How to Create Sentora Panel Subdomain for the installation?

[TGates]

You do not need to use their DNS. Since this is your main domain, you can set up the .tk using this tutorial: http://docs.sentora.org/?node=95

Then use Sentora's DNS to control everything else. 

All add-on domains will now use your nameservers as specified in the above tutorial.

I think it is more complicated (and impractical) to use the .tk DNS to point to your Google DNS then to Sentora's DNS.