Using custom installation page with invoking templates

Hey all. We have a custom installation page in which we first retreive the ticket fields for some settings. I’ve been trying to pass the Api Key as an param with invoking the template but I can’t get it to work.

My code now looks like this:

let subdomain = domain.replace(".freshdesk.com", "");
let apiKey = jQuery("#api-key").val().trim();

client.request.invokeTemplate('getTicketFields', {
   body: JSON.stringify({
       subdomain: subdomain,
       apiKey: apiKey,
                }),
              })

And the request.json looks like this:

{
    "getTicketFields": {
      "schema": {
        "protocol": "https",
        "host": "<%= subdomain %>.freshdesk.com",
        "path": "/api/v2/tickets",
        "method": "GET",
        "headers": {
            "Authorization": "Basic <%= encode(apiKey) %>"
        }
      }
    }
  }

But I unfortunately keep receiving the error: error while substituting templates

Anybody got any idea? I already checked if subdomain and apiKey have value and they do.