The best way to develop a drop-down-list controlled by a macro in a User Form for data-entry

My question is if there is a user form controlled by a macro, that when the data-entry user select a cell a macro is activated with a drop-down-list where the user can select an option and if in the options provided there is not a best one, then the user can add their own. There are several way to do this but I will like if somebody have a macro example, please provide it to me I will modify as required. I have several macros that do this process differently, one read the data to be selected from several cells from a worksheet and I have other that everything is in the macro. Please, let me know which way is the best

Thank you very much

Dennis Cantellops
US FDA