Correct way to pass headers to the HttpHeaders Object

I'm trying to figure out how to pass headers correctly when instantiating a HttpHeaders object in Angular 5 . I have the following code:

postApi(body: any, route: string) {
   return new Promise((resolve, reject) => {      
      this.http.post(url_api + route, JSON.stringify(body), this.options)
        .subscribe(res => {
           resolve(res);
        }, (err) => {
          console.log(err)
          reject(err);
        });
    });
}

When I try to instantiate the object HttpHeaders by passing an object as specified in the documentation . I get a 404 return (Actually the request was made to my API. But for some reason or error it does not recognize the route):

options = {
    headers: new HttpHeaders({
      'Content-Type': 'application/json',
      'x-api-key': 'xxxxxxxxxxxxxxxxxxxx'
    })
  };

However, if I create an empty object and then add my headers with append . Everything works perfectly and my route is found with no problem:

options = {
    headers: new HttpHeaders()
  };

postApi(body: any, route: string) {
    this.options.headers.append('Content-Type', 'application/json');
    this.options.headers.append('x-api-key', 'xxxxxxxxxxxxxxxxxxxx');
...
}

So I would like to understand if it is a mistake to instantiate an HttpHeaders object like this:

options = {
    headers: new HttpHeaders({
      'Content-Type': 'application/json',
      'x-api-key': 'xxxxxxxxxxxxxxxxxxxx'
    })
  };