Install SigningHub for SharePoint from other source
Browse your on-premises SharePoint Admin URL.
Click the "Site Contents" option from the left panel, and click the "add an app" option.
From the next appearing screen, click the "From Your Organization" option and then click SigningHub.
A confirmation dialog will appear, click the "Trust it" button.
SigningHub for SharePoint will be installed in your SharePoint account and displayed on the Site Contents screen.
1. The operator installing SigningHub for SharePoint, should have the SharePoint Admin rights (i.e. Tenant Administrator) on the site collection where the add-in is being installed.
2. Once the above installation is done, proceed to configure your SigningHub Account Settings, see details.