How to instantiate struct that is defined in different package from struct name and provide fields

Suppose I have some structs defined in package pkg:

package pkg

type Foo struct {
    FieldA string
    FieldB string
}

type Bar struct {
    FieldA string
    FieldB string
}

func (Foo) Show() {
    fmt.Println("Foo")
}

func (Bar) Show() {
    fmt.Println("Bar")
}

type Showable interface {
    Show()
}

Registry := map[string]Showable{
//not sure about value type^
    "Foo": Foo,     // staticcheck shows: not a type
    "Bar": Bar,     //
}
    

And I want to instantiate the structs dynamically; something like this:

package main

import "url/user/pkg"

func main() {
    foo := pkg.Registry["Foo"]{
        FieldA: "A",
        FieldB: "B",
    }
    
    bar := pkg.Registry["Bar"]{
        FieldA: "X",
        FieldB: "Y",
    }
    foo.Show()
    bar.Show()
}

The above clearly doesn't work.

Is it possible to achieve this? I am new to [tag:go]. I have looked at reflect, I have tried to build the Registry with pointers, pointers of empty instance, but couldn't figure out a way to do this.

Ultimately, I am trying to write a command line utility to change themes of certain programs. I have written program specific methods (like Show in above example), and I am trying to read the program specific params from a config.json file, and create the instances dynamically.

If I correctly understand what you are trying to achieve, here is
the way to do that:

registry.go:

package pkg

import (
	"fmt"
	"io"
)

type NewShowable func(r io.Reader) Showable

type Showable interface {
	Show()
}

type Foo struct {
	FieldA string
	FieldB string
}

func newFoo(r io.Reader) Showable {
	// Read config from r and construct Foo
	return Foo{}
}

func (Foo) Show() {
	fmt.Println("Foo")
}

type Bar struct {
	FieldA string
	FieldB string
}

func newBar(r io.Reader) Showable {
	// Read config from r and construct Bar
	return Bar{}
}

func (Bar) Show() {
	fmt.Println("Bar")
}

var Registry = map[string]NewShowable{
	"Foo": newFoo,
	"Bar": newBar,
}

main.go:

package main

import (
	"log"
	"os"

	"url/user/pkg"
)

func main() {
	f, err := os.Open("config.json")
	if err != nil {
		log.Fatalln(err)
	}
    defer f.Close()
	foo := pkg.Registry["Foo"](f)

	f2, err := os.Open("config2.json")
	if err != nil {
		log.Fatalln(err)
	}
    defer f2.Close()
	bar := pkg.Registry["Bar"](f2)
	foo.Show()
	bar.Show()
}