Populate a docusign custom envelope field

I create an envelope custom field from docusign portal

With the following code the envelope definition created and send the envelope for signing.

           EnvelopeDefinition envelopeDefinition = new()
        {
            EmailSubject = emailSubject,
            EmailBlurb = emailBlurb,
            Documents = documents,
            Recipients = recipients,
            Status = "sent"
        };
            EnvelopesApi envelopesApi = new(apiClient);
            EnvelopeSummary results = await envelopesApi.CreateEnvelopeAsync(data.AccountId, envelopeDefinition);

How can i add a value to the "SAP ID" ?

With the following code i make it work

I use the same name as i configured it in a docusign portal for a mapping.

            //Define custom field value "SAP ID"
            var envelopeCustomFields = new CustomFields
            {
                TextCustomFields = new List<TextCustomField>
            {
                new TextCustomField
                {
                    Name = "SAP ID",
                    Value = "12345-ABC",
                    Show = "true",
                    Required = "false"
                }
            }
            };

            EnvelopeDefinition envelopeDefinition = new()
            {
                EmailSubject = emailSubject,
                EmailBlurb = data.EmailBlurb,
                Documents = documents,
                Recipients = recipients,
                Status = (data.IsDraft) ? "created" : "sent",
                CustomFields = envelopeCustomFields
            };