mButtonSee.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                mPromoRef.addListenerForSingleValueEvent(new
ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                    String text = dataSnapshot.getValue(String.class);
                    mTextViewPromo.setText(text);
                }

                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) {

                    }

                });

            }
        });

    }
}