Thank you for contacting Syncfusion support.
We can add the custom annotation from the code behind using customStampItems API available in PDF Viewer. We have created sample for the same and shared in the below link,
Note: We can also add the stamp annotation using the option available in PDF Viewer default toolbar
Please try this and revert us with more details about your requirement if the provided detail does not meet your requirement. These details will be helpful for us to investigate further and assist you better.