Вы используете startWith
трубу:
const username$ = this.form.get('username')
.valueChanges
.pipe(startWith(this.form.get('username').value))
const password$ = this.form.get('password')
.valueChanges
.pipe(startWith(this.form.get('password').value))
combineLatest(username$, password$)
.pipe(
map(([username, password]) => ({username, password}))
)
.subscribe(res => console.log(res))
Stackblitz