Hi kas12345,
Refer below code.
public string InsertCustomer(CustomerModel customer)
{
    SqlParameter pId = new SqlParameter("@CustomerId", customer.CustomerId);
    SqlParameter pName = new SqlParameter("@Name", customer.Name);
    SqlParameter pCountry = new SqlParameter("@Country", customer.Country);
    var pOutId = new SqlParameter
    {
        ParameterName = "Id",
        DbType = System.Data.DbType.String,
        Size = 100,
        Direction = System.Data.ParameterDirection.Output
    };
    var result = this.Database.ExecuteSqlCommand("EXECUTE Customers_InsertCustomer @CustomerId,@Name,@Country,@Id", pId, pName, pCountry, pOutId);
    return Convert.ToString(result.Value);
}