HTTP post request unit testing in Angular 5. TypeError: newAlarms.generateUrlencodedParameters is not a function when

I would like to test an HTTP request in Angular 5, but it is not working. Here is the error message:

TypeError: newAlarms.generateUrlencodedParameters is not a function

and here is the code: It is a post request, and it working properly.

 public createAlarms(newAlarms: Alarm): Observable<boolean> {

    let headers = new Headers();
    headers.append('Content-Type', 'application/x-www-form-urlencoded');

    let body = newAlarms.generateUrlencodedParameters(this.getCurrentUser().token);

    return this.http.post(Api.getUrl(Api.URLS.createAlarms), body, {
      headers: headers
    })
      .map((response: Response) => {
        let res = response.json();

        if (res.StatusCode === 0) {
          return true;
        }
 }

and here is the test for it: This is the test for the post request. I used different articles to write it and perhaps that is why it is not working.

  describe('postAlarms()', () => {
    it('should create alarm',
      inject([AlarmsService, XHRBackend], (AlarmsService, MockBackend) => {
        const loginDetails = {
          alarmtype_id: 'gdfgr',
          alarmdesc: 'retst',
          alarmnumber: '521'
        };
        MockBackend.connections.subscribe((connection) => {
          connection.mockRespond(new Response(new ResponseOptions({
          })));
        });
        AlarmsService.createAlarms(loginDetails).subscribe((dd) => {
          expect(dd.status).toEqual('ok');
        });
      }));
  });

and this is my class:

export class Alarm {
    alarmtype_id: number;
    alarmnumber: String;
    alarmdesc: string;

    constructor(obj: any) {
        this.alarmdesc = obj.alarmdesc;
        this.alarmtype_id = obj.alarmtype_id;
        this.alarmnumber = obj.alarmnumber;
    }
    public generateUrlencodedParameters(token: string, id?: number): string {
        let urlSearchParams = new URLSearchParams();
        urlSearchParams.append('alarmnumber', this.alarmnumber.toString());
        urlSearchParams.append('alarmdesc', this.alarmdesc.toString());
        urlSearchParams.append('token', token);
        return urlSearchParams.toString();
    }}

For this testing, I follow this post