<form noValidate (ngSubmit)="onSubmit(login)" [formGroup]="login">
Username:<br> <input type="text" formControlName="username"><br>
<div class="error"
    *ngIf="login.get('username').hasError('required') && login.get('username').touched">
    Username required</div>

Password:<br> <input type="password" formControlName="password"><br>
<div class="error"
    *ngIf="login.get('password').hasError('required') && login.get('password').touched">
    Password required</div>

<br> <input type="submit" value="Login" [disabled]="login.invalid">

export class LoginComponent implements OnInit {
    login: FormGroup;
    response: ResponseInterface;

    constructor() {
    }

    ngOnInit() {
        this.login = new FormGroup({
      username: new FormControl('', [Validators.required, Validators.required]),
        password: new FormControl('', [Validators.required, Validators.required])});
    }

    onSubmit({ value, valid }: { value: LoginInterface, valid: boolean }) {
       console.log(value, valid);
    }
}

export interface LoginInterface {
  username: String;
  password: String;
}

export class DropdownosComponent {
   private list = [
      { id: 1, name: 'ONE' },
      { id: 2, name: 'TWO' },
      { id: 3, name: 'THREE'}
   ];
   private current: number = 1;

   private setCurrent(id: number): void {
      this.current = id;
   }
   public getCurrent(): string {
      return this.list.find((item: any) => item.id == this.current).name;
   }
}

<select #select [(ngModel)]="current"
(change)="setCurrent(select.value)">
   <option *ngFor="let item of list" [value]="item.id">{{item.name}}</option>
</select>

export interface LoginInterface {
  username: String;
  password: String;
  dropdown: String;
}