Trigger on form field when there is change in other form field angular5 form validation

Question

I have an issue of triggering an error in a input form, if other form field is changed.

myFormGroup = new FormGroup({
            'name': new FormControl('', [ Validators.required ]),
            'material': new FormControl('', [ Validators.required, NumberValidator.isaNumber ]),
            'prefix': new FormControl('', [ ]),
        });

I want to alert the user to enter prefix field only when he changes the material field. Can someone help me with this.

Thank you