Not able to set option set field value dynamically using workflow

I am creating a follow up Task from an existing Appointment using workflow.Along with other fields, I have one option set field containing same values and names respectively in both - Task and Appointment.I am able to select other fields values dynamically (Create:Task > Set Properties)  except above mentioned option set value. In form assistant, Look for:Appointment, but 2nd dropdown(list of fields) is blank.

I am using CRM 2015 online.

• I am able to set the static and default values for this option set.
• This option set field is mandatory in Task but optional in Appointment.
• Both option set fields have different 'Name'

Thanks for help in Advance!

Cheers!

Raushan