How to do it...

1. Create a new GameModeBase subclass called AttributeGameMode.
2. Update the AttributeGameMode.h file to the following: 

#pragma once

#include "CoreMinimal.h"
#include "GameFramework/GameStateBase.h"
#include "SlateBasics.h"
#include "AttributeGameMode.generated.h"

/**
 * 
 */
UCLASS()
class CHAPTER_14_API AAttributeGameMode : public AGameModeBase
{
 GENERATED_BODY()

 TSharedPtr<SVerticalBox> Widget;
 FText GetButtonLabel() const;

public:
 virtual void BeginPlay() override;
 
};

3. Add the implementation for BeginPlay within the .cpp file:

void AAttributeGameMode::BeginPlay()
{
    Super::BeginPlay();

    Widget = SNew(SVerticalBox)
        + SVerticalBox::Slot()
        .HAlign(HAlign_Center)
        .VAlign(VAlign_Center)
        [
            SNew(SButton)
            .Content()
        [
            SNew(STextBlock)
            .Text(TAttribute<FText>::Create(TAttribute<FText>::FGetter::CreateUObject(this, &AAttributeGameMode::GetButtonLabel)))
        ]
        ];
    GEngine->GameViewport->AddViewportWidgetForPlayer(GetWorld()->GetFirstLocalPlayerFromController(), Widget.ToSharedRef(), 1);

}

4. Also, add an implementation for GetButtonLabel():

FText AAttributeGameMode::GetButtonLabel() const
{
    FVector ActorLocation = GetWorld()->GetFirstPlayerController()->GetPawn()->GetActorLocation();
    return FText::FromString(FString::Printf(TEXT("%f, %f, %f"), ActorLocation.X, ActorLocation.Y, ActorLocation.Z));
}

5. Compile your code and launch the editor.
6. Override the game mode in World Settings to be AAttributeGameMode.

7. Note that, in a Play In Editor session, the value on the UI's button changes as the player moves around the scene: