MAXSTRING usually indicates that you're exceeding the maximum possible length of a string somewhere. Are you sure it's a problem with the %Stream.GlobalCharacter, and not a different string variable in your program? Global character streams shouldn't have that problem.

You can see what the maximum length of a string is on your system by opening a terminal and running:

write $SYSTEM.SYS.MaxLocalLength()

One of the properties of the %SOAP.WebClient class is HttpRequest which is an instance of %Net.HttpRequest. You might need to set the content type of that HttpRequest. So where you have "..ContentType", try "..HttpRequest.ContentType"?

The maximum URI size in Apache is usually 8,177 characters, but increasing that isn't the problem. Somewhere in your program, the value of your input is getting appended onto the URI, and it shouldn't be. Can you see anywhere that your program might be doing that?

I've had some time to try this now. Here are steps that worked for me:

set myrequest = ##class(%Net.HttpRequest).%New()
set myrequest.Server = "<server ip or domain here>"
set myrequest.Port = "<server port here if it isn't 80>"
set myrequest.Location = "</path/to/rest>"
do myrequest.EntityBody.Write("<your json here>")
do myrequest.Post()
set mydata = myrequest.HttpResponse.Data.Read()

At that point, the data returned in the response should be in mydata.

Depending on your specific API, you made need to take additional steps for authentication, and you may need to use myrequest.Get() or myrequest.Put() instead of myrequest.Post().

If you need to set parameters, you use the SetParam method of the HttpRequest. For example, if you're using the very most basic way to authenticate to a Cache instance, you do that by specifying a CacheUserName and a CachePassword as parameters as follows any time before your post/put/get:

do myrequest.SetParam("CacheUserName","<your username here>")
do myrequest.SetParam("CachePassword","<your password here>")

If your other system is also a Cache or IRIS server, there is a mirroring options called a reporting async that exists for this specific purpose. If you've got multiple servers and need to consolidate that data for reporting purposes, a reporting async can be a part of up to 10 mirrors to help you bring that data together, too.

Somewhere in your button tag, you have onselect= something. Buttons don't have an onselect, but even if they did, I'm guessing that's not the event you actually want. onselect happens when a user highlights text within a control, like in a text input.

If you're trying to set what happens when the user clicks the button, that's onclick.

If you're trying to set what happens when the user selects the button but doesn't click it (say by pressing tab until the button is highlighted) that's onfocus.

The %OnAfterCreatePage() method takes place after an instance of your page has been created on the server, but before it gets sent to the client, so it's really intended for server-side stuff.

Maybe instead you should be using the onloadHandler() method. That one runs on the client just before the page is displayed.

EnsLib.File.PassthroughOperation that lets you use timestamp variables, not EnsLib.File.PassthroughService. PassthroughOperation sends files, PassthroughService receives them.

I think you're looking for the %ArrayOfObjects class for this one. You'd create your objects with all of their value, ID, and type properties, then you'd create the array:

set array = ##class(%ArrayOfObjects).%New()

Then you set values of the array using the SetAt method:

do array.SetAt(downobject,"down")

Then to access a particular value, you use the GetAt method, then dot syntax to access the object's properties:

set myid = array.GetAt("down").id

Here's the %ArrayOfObjects class documentation.

Try replacing your while loop with this:

//set RET to a blank string to start to avoid issues with the first concatenation inside the loop
s RET = ""
while res.Next()
    {
     //Append a ~ and the value to RET
     s RET = RET_"~"_ res.GetData(2)
}
//The way we did this, RET will now start with a ~, which we'll want to remove
//This will look at RET, replace tildes with nothing, starting at the beginning, and only making one replacement
$REPLACE(RET,"~","",1,1)

//having done that, RET should now be, "description 1~description 2"