1. From your SharePoint 2019 Farm Server, open SharePoint 2019 Central Administration


2. On the Central Administration page, select Apps from the side panel. 


3. Under App Management, click Manage App Catalog.


4. Click the link under Site URL.


5. You will be redirected to your App Catalog.


6. On your App Catalog page, select Apps for SharePoint from the side menu.


7. On the Apps for SharePoint page, click New. 


8. A pop-up window will appear. Click Choose Files:


9. Select the SigningHub4SharePoint APP file from your machine.

10. Then, click OK.

11. On the "Do you trust" screen that appears, click "Deploy"

12. You will then see a message indicating that the upload was completed successfully.