@Directive({
    selector: '[ifValid]'
})
export class SubmitValidationDirective {

    @Output('ifValid') valid = new EventEmitter<void>();

    constructor(
        private formRef: NgForm
    ) {}

    private emitIfValid() {
        if (this.formRef.valid) {
           this.valid.emit();
        }
    }

}

<button (ifValid)="save()">

private save() {
    this.service.push(this.user, this.toCreate)
       .pipe(
           take(1)
       )
       .subscribe();
}