Make the field "other_c" required which is dependent on multiselect field

Hi,

How can we make the field "other_c" set to required which is dependent on the multi-select field "request_c".

The field "other_c" is visible only when "request_c" selects the option "Other". 

When the "other_c" field is visible, it should be made as a required field.  How can we do this?

Thanks