OnValidating for Form control event handler in D365

Hi All,

How to create On validating method for Form control event handler for extension form in D365 and how we can return Boolean value.

[FormControlEventHandler(formControlStr(RelTable, IsPrimary), FormControlEventType::Validating)]
public static void _IsPrimary_OnValidating(FormControl sender, FormControlEventArgs e)
{
FormRun element = sender.formRun();
}

Thanks and Regards

Prabhakaran.