Custom Objects

In the following piece of code I am inserting Check Buttons, thru a list of SignatureDetails. Here I am adding the entries to the 

DocumentBuilder documentBuilder = DocumentBuilder.NewDocumentNamed(document.FileName)
   .WithId(document.Id) // Ensure document ID is set correctly
   .FromStream(fs, DocumentType.PDF);

After the case statement is done with all records I added to the package builder 

packageBuilder.WithDocument(documentBuilder);

At the end I build the Package

var builtPackage = packageBuilder.Build();

In my current setup, I have a main custom object with two buttons: "Generate Contract" and "Send For Sign Off". The first button uses Conga Composer to generate the contract document and saves it to the main custom object. The second button creates the transaction pragmatically and navigate the user to the transaction record.

I want to relate transactions to my custom main object. I've tried populating the parentid field but I get an "Invalid Parent Id" error message. What are the steps to relate a transaction to a custom object record pragmatically?