How can I add a new boolean property to a Golang struct and set the default value to true?

I have a user struct that corresponds to an entity. How can I add a new property active and set the default value to true?

Can I also set the value of that property to true for all existing entities by some easy method?

type User struct {
	Id              int64     `json:"id"`
	Name            string    `json:"name"`
}

Bonus questions: I don't quite understand the syntax in the struct. What do the three columns represent? What do the JSON strings have ``around them?

//You can't change declared type.
type User struct {
    Id              int64     `json:"id"`
    Name            string    `json:"name"`
}
//Instead you construct a new one embedding existent
type ActiveUser struct {
	User
	Active bool
}
//you instantiate type literally
user := User{1, "John"}
//and you can provide constructor for your type
func MakeUserActive(u User) ActiveUser {
	auser := ActiveUser{u, true}
	return auser
}
activeuser := MakeUserActive(user)

You can see it works https://play.golang.org/p/UU7RAn5RVK

You have to set the default value as true at the moment when you are passing the struct type to a variable, but this means you need to extend that struct with a new Active field.

type User struct {
    Id              int64     `json:"id"`
    Name            string    `json:"name"`
    Active          bool
}

user := User{1, "John", true}

json:"id" means that you are mapping the json decoded object field to the field id in your struct type. Practically you are deserialize the json string into object fields which later you can map to their specific field inside the struct.