How do I use the MongoDB _id field in a form?

I have this struct:

struct Employee

string _id;
string deprt;
string email;
string pword;
string fname;
string lname;
string phone;
string paygd;
string photo;
string street;
string city;
string province;
string postcode;


The question is: what data type should the _Id be?

When I submit the form, I get this error:

BSON value is type 'objectID', expected to be one of [string, code, symbol]