Automatically Selecting 'on Click'

I have an event for the 'on click' of a command button
which begins:

Dim varitem As Variant
Dim supplst As String
For Each varitem In Me.SupplierQuery.ItemsSelected
supplist = supplist & ", '" & Me.SupplierQuery.ItemData(varitem) & "''
Next varitem

This is working to introduce individual names that I select from a list box into an 'IN' arguemnt later in the
code.

What I want, though, is for the 'on click' event to have already automatically pre-selected all the names
from the listbox before the ItemsSelected method is executed. In other words i do not want the user to
have to manually select the names from the listbox, because the names in the listbox already have been added by the user
previously as the supplier names that they would like to search. Can anyone help with this.