@amitabhanandy you forgot to invoke the HandleEvents() function.
Try to modify your code final lines as:
SymbolChanged = HasSymbolChanged();
HandleEvents();
CreateLabel( "LotSize" , 10, colorWhite, 10 , 50 );
CreateTextBox( id_LotSize, Trachine_Svar_Name_LotSize, 110 , 50, 80, 30, 10, SymbolChanged );
GuiButton( "SAVE / REFRESH", id_SaveRefresh, 10, 100, 200, 45, notifyClicked );
In general, I suggest you to use more _TRACE() instructions when debugging code. Adding one inside the HandleEvents() function immediately showed me that pressing the button did nothing since the event was not processed.
Moreover, there is no need to initialize the vars at the top of your formula as globals since they are "globals" by default in your code. Please, review this page of the guide.
Finally, while I welcome you to this community, I must also remember you to get your License Verified badge, or if you are still evaluating AmiBroker, post your next questions only in the specific Presale/Trial section of the forum.