The KB article you followed contains relevant instructions for mapping a sub-domain to a page group in a Sitefintiy 3.7 website. There are no additional steps that need to be taken.
Let's say your license file is for the root domain "mywebsite.com" and you have a website runnign on Sitefinity 3.7. If you map the sub-domain intranet.mywebsite.com to the same Sitefinity instance as per the instructions in the KB article, then you shouldn't receive a trial message when you request the sub-domain.
On the other hand, if you have registered a root domain "corporate.mywebsite.com" for the license key used for the particular Sitefinity 3.7 website. Then, you map another sub-domain to the same Sitefinity instance such as intranet.mywebsite.com, you will be receiving an occasional trial message for that second sub-domain because it is not a sub-domain of the licensed root domain. In case of intranet.corporate.mywebsite.com, then Sitefinity will not display a trial message because it runs with a key for the licensed root domain corporate.mywebsite.com.
If you continue receiving a trial message, please submit a support ticket indicating what your licensed root domain name is and also what the sub-domain is. Don't hesitate to share any additional information that you deem relevant in your case.
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