Basically, I have a list of stock symbols in column A that goes down about a thousand cells. I occasionally add to this list. Usually I just search (or use find feature) to see if I already have a given stock on the list, and if it is there then I ignore, but if it isn't I add it to the bottom.

I want to create a cell / box / formula on top where I will be able to enter a stock symbol (i.e., "IBM") and this box/cell/etc. will either return that the symbol is already on the list, or if it is not on the list will automatically add it to the list.

I have this so far which tells me only if a symbol already exists
=MATCH("IBM", A1:A500, 0) , which may be the wrong approach to start with, and that helps to see if a symbol is on the list, though I need something that will automatically add the symbol to the list if not found.

Thanks for help in advance!!