You can prefill placeholders with data on a prepared template document using our SignRequest Setup page in Salesforce.
If it's the first time you are creating your own Custom Object with prefilled_tags for a Salesforce Standard Object (Contacts, Leads or Opportunities, for example), you should open 'SignRequest SetUp' in Salesforce Lightning.
In the SignRequest SetUp page, you can find Advanced Settings, click on it.
After that, you can find two options. The first where you can prefill signers. And the second where you can prefill_tags for a prepared Template Document used for your SignRequests. For that, you can add the IDs of your placeholder with the value you want to give them, those values can be either Checkboxes, Texts or Dates. As shown in the example below:
Now when a document contains the tags above you can prefill them using the
prefill_tags field on a template document by referencing the 'id'. Note that tags are only prefilled, and thus visible, when the signer belonging to the prefilled tag opens the document. Only after a signer, with prefilled tags, has signed, those tags will also be visible to other signers opening the document. So for example, when the sender signer (index 0) has tags assigned (and prefilled using the api) and there is no particular order in which the signers sign (all at the same time), and the second signer opens the document the tags will not be visible. However, when the sender signer signs before the second signer open the document the tags will be visible while signing.