<form role="form" [formGroup]="loginForm" (submit)="login($event)" class="c-login__panel u-marg-30">
  ...
  <ika-text-input formControlName="username" label="USERNAME" [invalid]="username.invalid && (username.dirty || username.touched)"></ika-text-input>
  <div class="u-marg-30"></div>
  <ika-text-input formControlName="password" label="PASSWORD" type="password" [invalid]="password.invalid && (password.dirty || password.touched)"></ika-text-input>
  <a href="" [routerLink]="['/login/forgot-password']" class="txt--link txt-s--s txt-w--l col-primary">{{ 'FORGOT_PASSWORD_QUESTION' | translate }}</a>      <br>

  <button type="submit" class="o-btn o-btn--primary o-btn--big u-marg-30" [disabled]="loginForm.pristine || loginForm.invalid">{{ 'ENTER' | translate }}</button>
</div>

get username() { return this.loginForm.get('username'); }
get password() { return this.loginForm.get('password'); }
...
this.loginForm = _formBuilder.group({
  username: [null, Validators.required],
  password: [null, Validators.required]
});

this.loginForm.valueChanges.subscribe((el) => {
  el.forEach((control) => {
    if (control instanceof FormControl) {
      el.markAsDirty();
    }
  });
});