laravel required_if validation

how to make a field required depend on dropdown options,

I'm try to do with this rule in request file , but not get success

'verification_desc' =>  "required_if:employee_verification_sent_by,==,null",
'employee_verification_done_on' =>  "required_if:employee_verification_sent_by,==,2",

all fields are not required field and employee_verification_sent_by is a employee dropdown if employee are selected from dropdon this this two fields should be required.

via Chebli Mohamed