Input mask

[LEXCERM]

Hi,

I've created a simple input mask in a text box: AA,;;.

Users will be entering a comma delimited string within the textbox. However, the string can be of varying length:-
AA,
AA,2B ,D3,DD,
BB,CD,FF,
... and so on.

How can I achieve this so that when the first code is entered, the placeholder appears for continuation of text entry?

Thanks in advance.