... wanted to add History Vault to watch shows like Men who built America, Frontiersmen etc...

I'm located in the U.S., and I'm using Prime Video in a web browser at amazon.com/primevideo. — I search for The Men who Built America, to find this Prime Video page for that show:

https://www.amazon.com/dp/B009RURO0I/

There I see an orange button that says:
"Watch with History Vault. Start your 7-day free trial."

So, if I click that button, it should subscribe me to the channel.