If you download and install Prime, without licensing it (for the trial period) you are immediately in Express mode.
After that, at some point in time you can choose to acquire a (trial) license and from that point, your trial period starts. And of course, once that period has ended, you're back in Express.
> Is there a way to disable the premium function at the beginning so I don't save any sheets requiring the full Prime version?
Yes, in the options under "License" you may point Prime to a file without valid license informations. This usually turns a full version into the Express variant.
BTW, the Host-ID you mentioned above, usually means the MAC-address of your primary network interface.
The installation procedure is quite challenging and a PITA. During installation you provide the product code, which signals the PTC server, which is contacted during installation, that you are entitled for the full version. The installation routine now reads the MAC-address of the NIC in your machine and the PTC server creates and sends a license file which is bound to this MAC-address. Usually product codes are only valid for one or two installations and after that you need to contact PTC to help you create a new license file for a new machine or you are entitled to use the online license manager to do so yourself.
PTC lets us jump through quite a number of loops but as Fred wrote it got even worse, as PTC does not sell any perpetual licenses any more. This means we lose access to your IP if we don't pay the annual fee (for nothing, as there is no positive significant improvement visible concerning the "development" of Prime.
As I noted above, I was able to get a new license with my Product Code and MAC address. I used that in the installation and still did not obtain satisfactory results: no product code or licensed to name in the about Mathcad popup under help. Also, no help files...just the error noted above in the image of one of my posts.
Wish I could obtain a copy of Mathcad 11. I assume it will run on Windows 10, but maybe no. I have a physical version of Mathcad 6, but don't think it will run on Windows 10. I used to run it in Windows 7 under the "virtual machine" option. I have not found such an option in Windows 10.