A specific Tax Agent Declaration signature is not actually required for the following reason.

The permissions given to the Tax Agent by the Taxpayer that are stored in Access Manager (Linked to the Auskey when submitting) provide the tax agent with the right to submit the form on the client's behalf.

It generally works as:

So provided the Tax Agent has received a signed declaration from the Taxpayer which has been submitted to the ATO and stored in Access Manager then no actual agent signature is required to be appended to the form.

Further information may be found here