Create a (subtask) link to an existing issue in YouTrack

I'm writing an Action to create default tasks for a user story. This is how far I got:

var workflow = require('@jetbrains/youtrack-scripting-api/workflow');
var entities = require('@jetbrains/youtrack-scripting-api/entities');

exports.rule = entities.Issue.action({
  title: 'Create default subtasks',
  command: 'tt-create-subtasks',
  guard: function(ctx) {
    // Condition that must be met to enable the custom command:
    return ctx.issue.fields.Type.name == "User Story";
  },
  action: function(ctx) {
    var issue = ctx.issue;
    var newIssue = ctx.issue.copy(issue.project);
    newIssue.summary = 'API spoofing';
    newIssue.fields.Type = ctx.Type.Task;
//    var link = newIssue.links.add(issue);  ????
    workflow.message('Default task created under issue ' && issue.description);
  },
  requirements: {
    Type: {
      type: entities.EnumField.fieldType,
      Task: {}
    },
  }
});

How do I create a new link that makes newIssue a subtask of issue?

I've look through what code completions offers, the documentation for the Issue property, SO questions, the workflow code that already exists in YouTrack, but I'm stuck...

You should specify the issue link too. The following code should work (newIssue will be a subtask of issue):

newIssue.links['subtask of'].add(issue);

You can find the example here: https://blog.jetbrains.com/youtrack/2017/12/make-it-workflow-part-4-generating-new-issues/