Goal Summary
I use Item Invoices to bill and invoice our customers. I have customised my Invoice forms [Setup - Customise Forms - Invoices - Item] which I have saved as my own templates. The "Authorisation Code" field is not available for use when customising Invoices of type "Item", and we would like this to be exposed for use.
Research/Prep notes
I have included a lot of detail in order to communicate that I am not a technical struggler, and have taken many steps/Google searches/forum searches in order to try and solve this for myself. I have included pathways to show how I reach each field or window, and an explanation of how/why we take certain steps.
What I am trying to acheive:
I want to customise it further to meet our company's needs. I have tried using the 'Insert Fields' window (i.e. selected Fields from the top menu bar when customising the invoice form) and have succesfully used this to add some needed fields.
What the problem is:
The 'Insert Fields' window it is missing some fields related to an Invoice, specifically: fields which are present in the 'Sales - Edit Item' window [[Sales Command Centre - Sales Register - invoice] and/or the 'Receive Payments' window [Sales Command Centre - Sales Register - invoice - Payment].
This request is for the 'Authorisation Code' field to be available for use when customising "Invoice Templates" of type "Item".
I enter data into this field in the 'Applied Payment Details' window [Sales Command Centre - Sales Register - invoice - Payment, select Payment Methods as Credit Card, - Details] and wish to be able to provide my customers with their credit card's authorisation code when a payment is taken (which I enter into the field when payment is taken).
MYOB Receipt Templates
As we do not like MYOB's Receipt templates (they do not meet our needs - please do not suggest that we use these, I have already spent a lot of time on the phone with MYOB support, and the Customer Support member and I both decided that Receipts would not meet our company's needs), I have created an 'Invoice' template which we send out for paid invoices, as well as an 'Invoice' template which we send our for unpaid receipts. Nevertheless, I have looked into this in MYOB's 'Receipt' [Setup - Customise Forms - Receipts] but it is not a field available on Receipts either.
Our current workaround: we are currently entering this information in the 'Comment' field on the 'Sales - Edit Item' window [Sales Command Centre - Sales Register - invoice].
Our desired outcome
We would like the Authorisation Code field to be a field available for use when customising an invoice of type "item". This frees up the "Comment" field for use with actual comments instead of structured data like Authorisation Code.
Steps I have already taken to solve this for myself
Among other MYOB Discussions, I have already read the following Community discussions, they were either irrelevant or did not solve this specific issue:
Please do not assume one of the above resolves the issue 
Warm Regards
Eloise
PD Online
"'Authorisation Code' Field in Customised 'Item' Invoices"