The app will be unable to embed the SigningHub iFrame unless the correct required permissions is provided for within SigningHub. The following error is an example from the browser console log of the error that is expected if this is not done:

Refused to frame 'https://signinghub.com/' because an ancestor violates the following Content Security Policy directive: "frame-ancestors mysite.sharepoint.com".


To prevent this, follow these steps:

1. Login to SigningHub with a user that has the enterprise admin role.

2. Go to "Enterprise Settings" > "Integrations" and click "Edit" on the relevant integration.

3. Now select the "Settings" tab and specify the domain that can embed the iFrame under "Allowed Domains". 

NB!!! To allow any domain, leave the field blank. Note that this should not be done within a production environment as it raises security risks.