How to send value to blueprint from code?

I need to send value from code to blueprint, but my code don’t work.
I call it from another function. It works, I saw it in debug mode

UCLASS()
class HOME_API UChangeDialogBottomText : public UUserWidget
{
	GENERATED_BODY()
	
protected:
	UPROPERTY(EditAnywhere, BlueprintReadWrite) FString st_Name_Key;

public:
	UFUNCTION()
	void UpdateDialogBotton();
};

void UChangeDialogBottomText::UpdateDialogBotton()
{
	st_Name_Key = "core";
}

You want to define default value of your C++ variables in .h file.

Something like:

UPROPERTY(EditAnywhere, BlueprintReadWrite) 
FString st_Name_Key = "core";