Error running custom generator

Description

Running through the steps in this doc for how to create a custom generator. I've created the basic sample project in output/myLibrary, compiled it with mvn package, although I had to add

<repositories>
    <repository>
        <id>swagger-codegen-repo</id>
        <url>file://${basedir}/swagger-codegen-repo</url>
    </repository>
</repositories>

to the pom.xml file (as well as possibly other minor changes that were necessary) in order to get mvn package to compile successfully. The io.swagger.swagger-modules dependency also had to be added in order to get to the stage I am currently at.

I have made no changes to the MyclientcodegenGenerator.java file.

Swagger-codegen version

This occurs on both Windows (running in Power Shell) and Linux (Ubuntu Bash through WSL), with the latest version of swagger-codegen.

Command line used for generation

When I execute

java -cp output/myLibrary/target/myClientCodegen-swagger-codegen-1.0.0.jar:modules/swagger-codegen-cli/target/swagger-codegen-cli.jar \
  io.swagger.codegen.SwaggerCodegen generate -l com.my.company.codegen.MyclientcodegenGenerator \
  -i http://petstore.swagger.io/v2/swagger.json \
  -o myClient

(or the equivalent modified for Windows), the output is

Exception in thread "main" java.lang.RuntimeException: Could not generate supporting file 'SupportingFile:
        templateFile: myFile.mustache
        folder:
        destinationFilename: myFile.sample

Using the -jar method described elsewhere results in

Exception in thread "main" io.airlift.airline.ParseArgumentsUnexpectedException: Found unexpected parameters: [io.swagger.codegen.SwaggerCodegen, generate, -l, com.my.company.codegen.MyclientcodegenGenerator, -i, http://petstore.swagger.io/v2/swagger.json, -o, myClient]

Related issues/PRs

Appears to be very similar to #1566, the error that @xhh was having right at the end.

Suggest a fix/enhancement

It seems as if myFile.mustache is not being produced and/or placed in the right location by swagger-codegen-cli.jar meta, or that the README produced (and the guide on GitHub) do not adequately explain the need to create a myFile.mustache file and how to go about that.

Thanks in advance.

Start with the custom-generator instructions, output/myLibrary/pom.xml, MyclientcodegenGenerator.java, and the SwaggerCodegen generate and cli.jar meta entry points. Reproduce the command against petstore.swagger.io/v2/swagger.json and compare the result with issue #1566. Done means the documented custom generator builds and runs without the supporting-file or argument-parsing errors, with the required template and README guidance clear.