how to give default values in protocol buffer?

For proto3, custom default values are disallowed.

Update: The below answer is for proto2 only, proto3 doesn't allow custom default values.

Yes, you can give default values as you had written. default is optional for required, but for optional you have to mention the default values else type specific value is automatically assigned. Moreover you forgot to mention the type for Empid.

protobuf language guide states that

If the default value is not specified for an optional element, a type-specific default value is used instead: for strings, the default value is the empty string. For bools, the default value is false. For numeric types, the default value is zero. For enums, the default value is the first value listed in the enum's type definition. This means care must be taken when adding a value to the beginning of an enum value list.