Ok. So lets say you want to generate multiple invoices in single report.
You just need to pass an array in the json and iterate over it through templating engine.

{{#each items}}
  <h1>Invoice {{id}}</h1>
  ....
{{/each}}

Try something like this, we will add it to the docs.

rs.RenderAsync(new RenderRequest {
    Template = new Template()
    {
        Content = "Helo world",
        Engine = Engine.Handlebars,
        Recipe = Recipe.ChromePdf,
        Chrome = new Chrome
        {
            MarginTop = "2cm"
        },
        PdfOperations = new List<PdfOperation>()
        {
            new PdfOperation()
            {
                Type = PdfOperationType.Merge,
                Template = new Template
                {
                    Content = "header",
                    Engine = Engine.None,
                    Recipe = Recipe.ChromePdf
                }
            }
        }
    }
});

The jsreport-proxy is allowed only in the jsreport scripts
https://jsreport.net/learn/scripts

It is not supported in the templating engines, because it is anyway asynchronous and templating engine supports only sync functions.
If you put it to your global asset helpers, it gets extracted into the templating engine helpers and crashes.

📂📂📂
https://jsreport.net/blog/introducing-folders
https://github.com/jsreport/jsreport/releases/tag/2.3.0

The folders structuring features is now ready in jsreport 2.3.0.

the require('jsreport') creates completely new jsreport instance.
The require('jsreport-proxy') is a bridge from the script to the current jsreport instance.
You don't install it. It is kind of virtual mock we provide to the script.
Creating a new jsreport instance can have collisions with the current one.
You should not do that.

Does it fail also when using require('jsreport-proxy')?

Hm. Maybe the rendering of the dynamic template somehow crashes?
Do you use official docker image? What version?

Trying something like this in the docker image 2.2.0 and it works

const proxy = require('jsreport-proxy')

async function beforeRender(req, res) {
  const result = await proxy.render({ template: { content: 'another', engine: 'none', recipe: 'text' } })  
  req.template.content = result.content.toString()
}

Also in playground such script works
https://playground.jsreport.net/w/jan_blaha/CoDLg8j4

Thank you for the suggestion. We will consider it as an option for the future.

You get it wrong. I didn't create or use child template. Please read more about child templates in the docs.

Again, you don't have to create a template or use a child template. You can do everything in single post to /api/report.
In the limited way it looks like this:

{
  "template": {
    "recipe": "phantom-pdf",
    "engine": "none",
    "content": "<img style='display:none'... />",
    "phantom": {
      "header": "<img ... />"
    }
  }
}

If you would really want to use child templates. You need to first create a template using OData POST.
https://jsreport.net/learn/api#create

We don't say in the documentation you need to use child templates for headers. It is just one option.
In your request, you just need to take the same image you have in the phantomOptions.Footer and put it also to the templateObject.content.

See the demo. You can check how the request looks in the F12 browser tools.
https://playground.jsreport.net/w/jan_blaha/m20DFDtx