How can I set a checkbox custom field via API

I am setting custom fields in a template via the DocuSign API from Salesforce. This works great for text fields. I am having trouble setting a checkbox field to checked. What is the value to send to a checkbox field so it shows as checked?

Here is an example of code to set text fields:

DocuSignAPI.TemplateReferenceFieldDataDataValue fd0 = new DocuSignAPI.TemplateReferenceFieldDataDataValue();

fd0.TabLabel = 'Lead Name';

fd0.Value = c.FirstName + ' ' + c.LastName;

I've tried setting the Value to 'true', 'checked', 'Checked', 'TRUE'. None of these seem to work.

Thanks!