How to is covered in Classes & Representations training, available on SageU.
Herewith an example from online-help Managing CRUD Combined Objects | (sageerpx3.com)

$AQUERY_TRANS_AFTER
this.YCOMB.YGRP = [LNK_]YGRP
this.YCOMB.YNAM = [LNK_]YNAM
this.YCOMB.YUSER = GACTX.USER
Return

Hi Thanda,

This topic is covered under Development training.

You may attend the Development courses at Sage University which will cover this topic.

The next training is scheduled for November 2023:-

Please ensure that the training includes Representations and Classes, which is the topic you are interested in.

Hi Reiner,

I attended the development training (as well as advanced development - web services) in 2019 and received the manual for it as well. However neither the training nor the manuals had anything on classes and representations. 

Hi Thanda,

Are you working for a Business Partner? If yes, are you certified in the Development module?

If both the answers above are yes. I would suggest logging an issue with Sage X3 Customer Services South Africa for one of our Developers to assist you in more detail.

As can be seen below Appendix A includes Representations and Classes. I am not certain why your course did not include this section.

Hi Reiner,

My manual does end with a heading "11.0 Understanding Classes and Representations" but there is no content after that - it ends with the heading only.

I will maybe have a look at contacting Sage.

Hi Nzou,

Thank you - this seems to be what I am after. 

Hi Thanda,

What is your surname, please? Also, provide me with your email address.

Should our records indicate you attended this training, I will provide you with this section via email.

Thank you and have a super day!

Hi Reiner, 

My surname is Mabaso. My full name is Thanduxolo Mabaso. 

Thank you Thando.

Also, provide me with your email address please.

It's [email protected] 

I may have used my work email which is [email protected] 

Thank you Thanda, will get back to you shortly.

Hi Thanda,
I will email you the section on Representations and Classes our record shows that you have attended the Dev Basics training in September 2019.