how should we set a multi select field? it was overriding the existing value on the field. so then I tried setting the field with the existing and new value, but get this error:

You have entered an Invalid Field Value null\u00059271228
var so_record = context.newRecord;
            var new_so_id = new Array();
            new_so_id = so_record.id;// NEW VALUE

var existing_so = case_update.getValue({
                fieldId: 'custevent_case_sales_order'
            }); //EXISTING VALUE

case_update.setValue({
                fieldId: 'custevent_case_sales_order',
                value: [existing_so, new_so_id]
            });

\u0005

is what is used internally as a separator for multi select fields

Ah right so I'd need a separator. Would you say that's the correct approach, as in getting the existing values and resetting them or it's not required?

depends on the context, in my experience multi select fields are unreliable in client script

I'm trying to set it on afterSubmit. My existing so returns the internal id of the so and the type is object.

one of your assumptions is wrong

I was able to set the multi-select field without a separator: existing_so.push(new_so_id); the existing so type returns object, yet I was able to use an array function (push) on it. Seems a bit odd.

typeof [] === 'object'

ah right, fine