How to add background text to signature pop up on OTM Web App ?

Is there a way to add background text for signatures captured on the OTM web app ? Our client wishes to use it as a legally binding electronic signature authorizing the release of shipment at drop off stop. Today a white page pops up and receiver is able to sign , however, client stats that this cannot be used as an authorization of release since there is no text against which the receiver is signing and confirming receipt.