The OOB Esignature Approval functionality is available on Classic UI, wherein while approving, the approver need to authenticate using username and password.
The same functionality is not available on the Service Operations workspace (SOW) OOB as the UI page displayed for credentials is not compatible with workspace.
This functionality can be achieved on workspaces through declarative actions and making the authentication page compatible with workspace.
I have made this functionality available on ServiceNow Share as Update set : Link
