How to append custom attribute to Golang struct like $append on a Laravel model?

I am trying to add a custom attribute to my Golang struct just like how I usually add custom attribute on a Laravel model using the $appends variable.
<br>
This is the code:

package models

import (
	"time"
)

type Dummy struct {
	ID          string `json:"id" gorm:"primary_key"`
	Description string `json:"description"`
	Image       string `json:"image"`
	ImageUrl    ImageUrlDummy
	Number      int       `json:"number"`
	UserId      string    `json:"user_id"`
	User        User      `json:"user"`
	CreatedAt   time.Time `json:"created_at"`
	UpdatedAt   time.Time `json:"updated_at"`
}

func ImageUrlDummy() string {
	return "test"
}

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However, the ImageUrlDummy inside the struct does not work, it return error such as:

ImageUrlDummy (value of type func() string) is not a type

<br>
How do I achieve this same code from Laravel to Golang?

class Dummy extends Model
{
    protected $appends = ['image_url'];

    public function getImageUrlAttribute(){
        return "this is the image url";
    }
}

<br>
Please pardon me I am still learning Golang, thank you

You are not far off..

Change your struct to (remove ImageUrlDummy, fix json tag for Image):

    type Dummy struct {
        ID          string `json:"id" gorm:"primary_key"`
        Description string `json:"description"`
        Image       string `json:"image"`
        Number      int       `json:"number"`
        UserId      string    `json:"user_id"`
        User        User      `json:"user"`
        CreatedAt   time.Time `json:"created_at"`
        UpdatedAt   time.Time `json:"updated_at"`
    }

Then define a method with a receiver of type Dummy pointer

    func (d *Dummy) ImageUrl() string {
        return "test"
    }

Example with a few more things: https://play.golang.com/p/olGSFhBgqkG