Word plug-in cannot connect to on-premise Creatio (HTTP only, internal bank network)

Hello!
 

We are trying to connect the MS Word to Creatio, but the connection always fails with a secure connection error, even when correct login and password are used.

Everything works only inside the bank’s local network, with no internet access at all.
So we are wondering:

 Is HTTPS absolutely required for the Word Add-in to work, even in a local environment? Or is there any supported way to make the add-in connect without HTTPS?

Any official guidance or documentation link would be very helpful.
Thank you!