Because v1.9.1 is a legacy version (modern versions are 1.14+), the "proper" way to get this specific build is to use the official version-rollback feature provided by Paradox Interactive. Step 1: Purchase and Installation
The Field Marshal Edition features are tied to your account's DLC list. Ensure the launcher shows version 1.9.1.
Are you trying to run a specific that only works on version 1.9.1?
Note: You can only play with others who are also specifically on v1.9.1. A Note on Third-Party Downloads
Click "Multiplayer" in the main menu. You can use the Steam "Invite Friend" feature or the Server Browser.