Output data depended on Listbox selection

Hi All,

I want to have a button on a form fill in a column if a value in its corresponding row has been selected as part of a multi list box.

I've uploaded an example sheet. For example if I selected "4" and "11" in the list box and pressed the Add Lines button I want a "1" to appear in column L for all the lines where there is a 4 or 11 in column A.

Any help would be greatly appreciated, Thanks

Hopefully there are no rules against bumping threads here. Any help with this would be greatly appreciated. I'm getting lost in VBA and getting absolutely nowhere!

Thanks