Docusign Template is no merging with APEX code

I send in the opportunity id into mySourceId into the routine and run through what should be the standard Docusign APEX API routines. We have a button that completes this merge using the standard Docusign routines, but are trying to get around all the extra button clicks the users are having to endure.

I have a template that is created and available on Docusign with Salesforce related Custom Field tags on the document. Is there something from the documentation that I'm missing?

        //create the emptyenvelope connecting it to the SF object
        myEnvelope = dfsle.EnvelopeService.getEmptyEnvelope(new dfsle.Entity(**mySourceId**));

        //use the Recipient.fromSource method to create the Recipient
        dfsle.Recipient myRecipient = dfsle.Recipient.fromSource(sender.name, // Recipient name
                                                                 sender.eAddress, // Recipient email
                                                                 null, //Optional phone number
                                                                 sender.role, //Role Name. Specify the exact role name from template
                                                                 new dfsle.Entity(**mySourceId**)); //source object for the Recipient
        //add email detail for the envelope
        myEnvelope = myEnvelope.withEmail(settingsDocusignTemplate.get(sourceName).email_subject__c, settingsDocusignTemplate.get(sourceName).email_body__c);

        //Could provide automatic notifications as well based off of criteria
        final boolean dfsle_reminder = settingsDocusignTemplate.get(sourceName).reminder__c;
        final integer dfsle_remindAfterDays = Integer.valueOf(settingsDocusignTemplate.get(sourceName).remindAfterDays__c); //wait before sending reminder
        
        //add Recipient to the Envelope
        myEnvelope = myEnvelope.withRecipients(new List<dfsle.Recipient> { myRecipient });

        //create a new document for the Envelope
        dfsle.Document myDocument =     dfsle.Document.fromTemplate((dfsle.UUID)templateInfo.get('UUID'), // templateId in dfsle.UUID format
        (String)templateInfo.get('name')); // name of the template

        //add document to the Envelope
        myEnvelope = myEnvelope.withDocuments(new List<dfsle.Document> { myDocument });

    try {
            
        System.debug('\tSending Envelope');
        dfsle.EnvelopeService.sendEnvelope(myEnvelope, // The envelope to send
                                                    true); // Send now?
    } catch (dfsle.DocuSignException docusignExcpt) {
        ErrorLogUtil.createErrorLogData(docusignExcpt,CLASS_NAME,METHOD_NAME,null,null);
    } catch (Exception excp) {
        ErrorLogUtil.createErrorLogData(excp,CLASS_NAME,METHOD_NAME,null,null);
    }

Try adding this to your code just after "create a new document for the Envelope"

String opptyStr = (String) myOpportunity.Id + '~Opportunity';
        dfsle.CustomField myField = new dfsle.CustomField ('text', 'DSFSSourceObjectId', opptyStr, null, false, false);

//add document to the Envelope
myEnvelope = myEnvelope.withDocuments(new List<dfsle.Document> { myDocument })
.withCustomFields(new List<dfsle.CustomField> {myField});

Technically merge fields are not supported but we can get around this issue by using custom fields with this code.