Defaults for Multiselect fields in Suitelets

I an working with a suitelet and have a multiselect field which is configured to use an existing record type to build the list, i.e.
[INDENT]myField = form.addField('multiselectfield', 'multiselect', 'Select one or more options', myRecordType);[/INDENT]

This shows the list fine and the user can select multiple values.  When I retrieve the field from the request parameters (when the user submits the page), I get back a single string with the selected values separated by ' '.

So, I can show the list, use the list, retrieve the values - but if I then redisplay the page, how do I preserve the current selections?  For other field types, I just need to set the field 'default value' to the value read from the request parameters, but for a multiselect field this does not seem to work.