form: FormGroup;

ngOnInit() {
this.form = this.fb.group({
value: ['', [ Validators.required ], this.checkIfValueIsValid]
});
}

checkIfValueIsValid(control: AbstractControl) {
return new Promise(resolve => {
if (control.value === 'invalid_value') {
resolve({ valueIsValid: true });
} else {
resolve(null);
}
});
}
<form [formGroup]="form">
<div class="select-group">
<label>Invalid/Valid</label>
<cui-select formControlName="value">
<cui-option [ngValue]="'invalid_value'">
invalid
</cui-option>
<cui-option [ngValue]="'valid_value'">
valid
</cui-option>
</cui-select>
</div>
</form>