Set Field [YourTable::YourField ; List ( YourTable::YourField ; "newValueHere" ) ]
Will append the new value to the end of the other values already present in the field.
List ( YourTable::YourField ; "newValueHere" ) can also be used with Replace Field Contents to do a batch update of your found set if you select the calculation option.
one more related question, which is basically the opposite scenario: is there also something to the equivalent of "remove from list"? Here the scenario is the opposite: for a found set of records I'd like to remove a specific value from a field that already contains multiple values.
With care, Substitute can be used to remove specified list items.
Let ( List1 = Substitute ( ¶ & YourTable::YourField & ¶ ; "¶newValueHere¶" ; ¶ ) ;
Middle ( List1 ; 2 ; Length ( List1 ) - 2 )
This approach makes sure that deleting "Apple" from the list doesn't change "Applesauce" into "sauce". ¶ stands for a carriage return and there's a button to add it to your calculations in the specify calculation dialog box. This expression puts an extra ¶ at the beginning and end of the text, long enough for substitute to remove the specified item, then removes them.
Substitute is case senstive so watch your capitalization.
BTW, there's a pretty cool trick you can use with these calcuations. you can set up a portal of items with the fields in the portal formatted as buttons that perform a script that inserts the item if it's not in the list and deletes it if it is. Conditional formatting can be used to show which items are currently in the list. This produces what is essentially a check box group, but one that can have a scroll bar if there are more items than there is space to list them all at the same time...
Fabulous! Got to the remove stuff to work. I'll look into the portal solution you mentioned to fancify it...