Docusign Unable to Update Form Fields or Custom Fields in Template Document

I am trying to update the fields in the template attached documents. I have tried to use the below code to update the custom fields , but not able to see any changes on the document sent to the user.Is it possible to change the custom fields in the template programmatically ?

 TemplateRole tRole = new TemplateRole
            {
                Email = "senderemailid@gmail.com",
                Name = "ABHAY DHAR",
                RoleName = "Test1"
            };

            tRole.Tabs = new Tabs();
            tRole.Tabs.TextTabs = new List<Text>();
            Text textTab = new Text();
            textTab.TabLabel = "Dummy_Custom";
            textTab.Value = "RedPlanetNote2";
            tRole.Tabs.TextTabs.Add(textTab);


            List<TemplateRole> rolesList = new List<TemplateRole>() { tRole };

            // add the role to the envelope and assign valid templateId from your account
            envDef.TemplateRoles = rolesList;
            envDef.TemplateId = "964300cb-9ccf-471f-8d9d-5ad894e8d99f";
           // envDef.CustomFields = docCustomFields;
            // set envelope status to "sent" to immediately send the signature request
            envDef.Status = "sent";

            // |EnvelopesApi| contains methods related to creating and sending Envelopes (aka signature requests)
            EnvelopesApi envelopesApi = new EnvelopesApi();
            EnvelopeSummary envelopeSummary = envelopesApi.CreateEnvelope(accountId, envDef);