Appending string value in cell with new unique selection from its drop-down list

[logicalchoice]

Hi, I'm very new to VBA programming and I'm not yet able to put together a macro I need . I'm not quite sure even where to get started. I'd like to create VBA Macro that allows a blank cell to be set a value via its drop-down list then take each subsequent value selected via the same drop-down lists and append new string values only to the current the string of the same cell, and separating each new entry by ",". Essentially the drop-down list allows pre-determined values to be conveniently selected, however in some instances multiple of those values from the drop-down list applies and want them also displayed. Ideally would also be great if any text values typed in the cell that are not already available in the drop-down list, to be added to the drop-down list. Any help with this would be much appreciated. Thanks in advance .