this.form = fb.group({
        username: ['', Validators.compose([Validators.required])],
        fullName: ['', Validators.compose([Validators.required])],
        password: ['', Validators.compose([Validators.required])],
        confirmPassword: ['', Validators.required],
    }, {validator: matchingPasswords('password', 'confirmPassword')});

export function matchingPasswords(passwordKey: string, passwordConfirmationKey: string) {
return (group: FormGroup) => {
    let password = group.controls[passwordKey];
    let passwordConfirmation = group.controls[passwordConfirmationKey];
    if (password.value !== passwordConfirmation.value) {
        return passwordConfirmation.setErrors({mismatchedPasswords: true})
    }
}

<div class="form-group">
        <input [formControl]="confirmPassword" class="form-control checking-field" type="password">
        <span class="help-block text-danger" *ngIf="form.get('password').touched && form.get('password').hasError('required')">
</div>
<div class="form-group">
        <input class="custom-control-input checkbox-main" type="checkbox" [(ngModel)]="policyButtonValue" [ngModelOptions]="{standalone: true}" ngDefaultControl>
        <span class="custom-control-indicator"></span>
</div>