This doc is a great place to start:
https://docs.intersystems.com/irisforhealthlatest/csp/docbook/DocBook.U…

You can use the %Dictionary classes to get details of the class and methods:

USER>do $System.SQL.Shell()
SQL Command Line Shell
----------------------------------------------------
 
The command prefix is currently set to: <<nothing>>.
Enter <command>, 'q' to quit, '?' for help.
[SQL]USER>>SELECT FormalSpec FROM %Dictionary.MethodDefinition WHERE parent='My.Test.Class' AND Name='TestMethod'
1.      SELECT FormalSpec FROM %Dictionary.MethodDefinition WHERE parent='My.Test.Class' AND Name='TestMethod'
 
FormalSpec
param1:%String,param2:%Numeric
 
1 Rows(s) Affected
statement prepare time(s)/globals/cmds/disk: 0.0033s/321/1482/0ms
          execute time(s)/globals/cmds/disk: 0.0006s/3/385/0ms
                          cached query class: %sqlcq.USER.cls14
---------------------------------------------------------------------------
[SQL]USER>>

A couple of things...

You'll want to use "O" (output) mode instead of "I" (input) mode:
write $zconvert("Flávio","O","HTML")

However, I see that this still doesn't replace the "á". But if I add an "&" it replaces it with "&" as expected:
write $zconvert("Flávio&","O","HTML")
The table at the bottom of this section in the documentation lists which characters are encoded into entities in HTML mode.

As an alternate approach, you could use the Lookup() function to check if the value exists in a lookup table. The advantage is that adding entries to a lookup table is easier/cleaner than updating your DTL with every new entry.

Two more options. Haven't tested how they perform.

set lst = ""
for i=1:1:someNumber {
    set $LIST(lst,i)=myval
}
set str = $listtostring(lst,",")

set lst = ""
for i=1:1:someNumber {
    set $PIECE(lst,",",i)=myval
}

Here's a working example using <img> in a <table>. I don't think you can put an <img> inside an <item> or inside another <img>.

<table orient="col">
            <block width="40%">
                    <img src="http://localhost/logo.jpg" contentHeight="0.56in" contentWidth="2.33in" style="text-align:left"/>
            </block>
            <block width="40%">
                <table orient="col">
                        <item field='#(..pagetitle)#'/>
                </table>
            </block>
            <block width="20%" style="text-align:right;">
                  <item field='#(..subtitle)#'/>
             </block>
</table>

Removing my reply as this was already answered.

It would be good to have a look at the final value of tSQL in performUpdate.

Here is the section in the docs that explains why this is happening and why @Eduard Lebedyuk asked if that class is persistent.
 

Business Process Execution Context

The life cycle of a business process requires it to have certain state information saved to disk and restored from disk, whenever the business process suspends or resumes execution. This feature is especially important for long-running business processes, which may take days or weeks to complete.

A BPL business process supports the business process life cycle with a group of variables known as the execution context. InterSystems IRIS automatically save the variables in the execution context and restores them each time the BPL business process suspends and resumes execution.

ObjectScript includes utilities for generating GUIDs:
https://docs.intersystems.com/irisforhealthlatest/csp/docbook/DocBook.U…

In particular, have a look at $System.Util.CreateGUID():
https://docs.intersystems.com/irisforhealthlatest/csp/documatic/%25CSP…

The HL7 organization offers training:
http://www.hl7.org/training/index.cfm

And there are other organizations that offer HL7 classes as well:
https://www.pathlms.com/hl7/courses/8159
I haven't taken these courses myself so I can't say if these are good or not.

If there aren't any delimiters in your input string, then this will do it:

set result="233434345543433400110"
for i=1:4:$LENGTH(result) {
    write $E(result,i,i+3),!    
}

Output:

2334
3434
5543
4334
0011
0

File Specs for services support wildcards, but not the complex time stamp specifications that are used for outputting filenames from an operation.

File Spec
Filename or wildcard file specification for file(s) to retrieve. For the wildcard specification, use the convention that is appropriate for the operating system on the local InterSystems IRIS Interoperability machine.

https://docs.intersystems.com/irisforhealthlatest/csp/docbook/Doc.View…

I haven't tested the code below, but in your subclass you can override MakeRequest(), call the original superclass version of MakeRequest() and then you should be able to set the header before returning from your custom method.

Something like this:

Method MakeRequest(pRequestMethod As %String, pPayload, pRequestPath As %String, pQueryString As %String, ByRef pHeaders) As %RegisteredObject
{
    set ..%HttpRequest = ##super(pRequestMethod, pPayload, pRequestPath, pQueryString, .pHeaders)
    do ..%HttpRequest.SetHeader("Some Header Name", "Some Header Value")
    return ..%HttpRequest
}