Delegating signing - why doesnt delegates signature show they have been delegated to sign on behalf?

Community - need your help here please. 

Our company directors want to delegate signing to their assistants. however they dont want the signature on the document to show the Assistants name, but show that they are signing as a delegate on behalf of the original signer (the director). 

Can anyone tell me if this can be done please?

Thankyou!