Go AppSource Website
Open the Secured Signing plugin page where you can view further details.
We assume that you are already logged in to AppSource with a Dynamics365 account with administrator permissions.
Microsoft will ask to choose which Dynamics 365 instance you want to install.
Microsoft will install the Secured Signing plugin. You can check the progress in the Dynamics 365 Administration Center.
The D365 Admin user can find the Secured Signing plugin in "Settings" > "Solutions".