The first thing you have to understand is that Step types are just large data types. You can always go to the definition and see the subproperties. Then you can use them as property objects to set and get different subproperties.
What makes one step type different from another is simply the subproperties. Like I mentioned above the subproperties can be found by going to where the step type is defined. This is generally in one of the INI files in the type pallette. OR you can place down a step of that type in TestStand and on the Properties tab choose the Property Browser. Everything in the property browser is relative to Step.
However, most of the step type subproperties are also called out in the TestStand help. For example if you open the TestStand help and on the Index tab type "Message Popup step Type" it will bring up the Message Popup Step page. It then shows all of the subproperties and what they are used for. Specific to your example you will see Step.MessageExpr. So in LabVIEW you can use an invoke node to call AsPropertyObject on your step. Then another invoke node to call SetValString. If the type is an Expression you have to include all of the proper formatting such that it will evaluate properly as an expression. In this case the quotes need to be injected as well. I've attached a simple example.
I think once someone really figures out the PropertyObject in TestStand and how to use the API for it then they have it made. It is what drives TestStand in my opinion. Everything revolves around property objects.
As a side note: all of the tabs that you use to configure a step are simply setting those sub properties. If you knew what you were doing you could go directly to the Property Browser and simply set a bunch of stuff in there. Technically thats what you are trying to do through the API.
For some steps NI was kind enough to create direct API calls depending on the module type or adapter type. That's why with the previous example we used the API. Always use the API if it is available. If not then use PropertyObject. The reason the API is better is because it might be setting multiple subproperties. And that is true for the Sequence Call paths. Because not only are you setting the Step.TS.SData.SeqName sub property but you are also setting the SpecifyByExpr to False. Most of the time if the subproperty is in the TS and especially the SData containers then there are API calls for them.
Hope this helps,
