Convert sub struct to string and back

I am having some trouble with the UnmarshalJSON and MarshalJSON i am not sure i understand them correctly or atleast i have an error that i have not been able to spot and in my 3 hours of googling i haven't been able find, so here goes.

I have the following User struct

type User struct {
	ID        uuid.UUID            `gorm:"primaryKey,type:string,size:36,<-:create" json:"id"`
	Username  string               `gorm:"unique" json:"username"`
	Password  PasswordHash         `gorm:"type:string" json:"password"`
	CreatedAt time.Time            `gorm:"autoCreateTime:milli" json:"created_at"`
	UpdatedAt time.Time            `gorm:"autoUpdateTime:milli" json:"updated_at,omitempty"`
	DeletedAt gorm.DeletedAt       `gorm:"index" json:"deleted_at,omitempty"`
}

which when i try to convert into JSON, with the json.Marshal function from the build in encoding/json library, i get the following:

{"id":"3843298e-74d4-4dd7-8eff-007ab34a4c19","username":"root","password":{},"created_at":"0001-01-01T00:00:00Z","updated_at":"0001-01-01T00:00:00Z","deleted_at":null}

I expected some thing like:

{"id":"3843298e-74d4-4dd7-8eff-007ab34a4c19","username":"root","password":"$argon2id$v=19$m=4194304,t=1,p=64$Z9EFSTk26TQxx+Qv9g58gQ$4At0rvvv9trRcFZmSMXY0nISBuEt+1X8mCRAYbyXqSs","created_at":"0001-01-01T00:00:00Z","updated_at":"0001-01-01T00:00:00Z","deleted_at":null}

i can't seem to get the password field to be a string of the hashed password.
even when i have the following methods on the PasswordHash struct

type Params struct {
	memory      uint32
	iterations  uint32
	parallelism uint8
	saltLength  uint32
	keyLength   uint32
}

type PasswordHash struct {
	hash   []byte
	salt   []byte
	params Params
}

func (h *PasswordHash) String() string {
	b64Salt := base64.RawStdEncoding.EncodeToString(h.salt)
	b64Hash := base64.RawStdEncoding.EncodeToString(h.hash)
	return fmt.Sprintf(
		"$%s$v=%d$m=%d,t=%d,p=%d$%s$%s",
		algoName,
		argon2.Version,
		h.params.memory,
		h.params.iterations,
		h.params.parallelism,
		b64Salt,
		b64Hash,
	)
}

func (h *PasswordHash) UnmarshalJSON(data []byte) error {
	var v string
	if err := json.Unmarshal(data, &v); err != nil {
		return err
	}
	params, salt, hash, err := decodeHash(v)
	if err != nil {
		return err
	}
	h.params = params
	h.salt = salt
	h.hash = hash
	return nil
}

func (h *PasswordHash) MarshalJSON() ([]byte, error) {
    //return []byte(h.String()), nil
	return json.Marshal(h.String())
}

So i guess my question is, shouldn't the MarshalJSON be called on the PasswordHash struct when trying to convert the user to JSON? and if so how come i can't seem to get it to be a string value?

You have MarshalJSON defined with a receiver of *PasswordHash, but your value is type PasswordHash. Change the receiver to PasswordHash and it works as expected: https://go.dev/play/p/WukE_5JBEPL