how to make customfield is required on workflow transition if cascading custom field values == 'xxx'

[saroja muninaga]

HI team ,

Actually i want to make customfield is required on particular workflow transition and if 
cascading custom field value equal to 'XXX'

Drop down custom field required on below conditions

-> While  workflow is changing to In Progress if cascading custom field is equal to  some 'XXX'

i have tried behaviors to achieve this . but it is not working . 

kindly anyone can suggest me on this 

Thanks,
Saroja