DNS Issues

I am having a DNS issue with our website with our internal computers. We have an internal Active Directory Domain with integrated DNS. When I point www.ourdomain.com to ext-cust.squarespace.com with a CNAME record it is not finding the site for internal computers. This entry is in the internal DNS. It resolves externally, as the external DNS for queries outside the company is on Squarespace. All internal computers use the AD DNS with forwarding. I have used a split or dual DNS for internal /external queries for years with no issues. I am doing this with two other companies both with AD Domains with no issues. One is hosted with bluehosting and the other is on a internal IIS server that is natted with loopback. The latter use to be on Wix and it had a CNAME value similar to Squarespace and it worked fine. The bluehosting points directly to an IP A record. Squarespace is the only hosting provider that I cannot get this to work with. This is the first time I have tried this with Squarespace. The internal computers have to use the internal DNS due to the Active Directory domain. Is anyone using a similar setup and know a way to get this to work with Squarespace? Any help is appreciated.

I found out the cause, but I'm sure why this is happening on Squarespace. When I type in www.domain.com from anywhere including and external computer off premise, the web address is going to domain.com. The www is stripped from the URL. Tested with multiple browsers and OS versions. This is causing my internal on-premise DNS issue. Is this something done by Squarespace? If so, is there a way to correct this in the DNS control panel?

I do not have the 'Use www prefix' setting under the primary domain. Does maybe Developer mode need to be turned on?

I found it. It is actually a dropdown vs the radio button. That also solved the issue with the on-premise DNS. Thanks.