Can't prefill Signer Fields

Can't prefill Signer Fields

I am trying to create a doc where the customer needs to enter their information like name, address etc. It might be the case that we took their data beforehand and stored it in the CRM already. If that is the case, we want them to see their data and just correct if necessary.  

However, there is no possibility to prefill signer fields in Zoho Writer: