Outlook web add-in How to get customer properties via EWS in JS

Question

How to get customer properties via EWS in JS if I saved them via Outlook JS. API

1. I saved custom properties using the following JS code:

      Office.context.mailbox.item.loadCustomPropertiesAsync(callback);
   
         function callback(asyncResult) {
         var customProps = asyncResult.value;
         customProps.set("isSync", "test");
   
        // Save custom properties via JS API
        customProps.saveAsync(saveCallback);
       }
   

2. Then I would like to get this custom prosperity via EWS.

Case 1:

   '<t:ExtendedFieldURI PropertySetId="cecp-my manifest" PropertyName="isSync" PropertyType="String"/>' 

Case 2:

   '<t:ExtendedFieldURI DistinguishedPropertySetId="PublicStrings" PropertyName="isSync" PropertyType="String"/>' 

Both approaches does not work. How should I describe <t:AdditionalProperties>?

At the same time everything works if I use .Net:

 ExtendedPropertyDefinition extendedPropertyDefinition = new ExtendedPropertyDefinition(DefaultExtendedPropertySet.PublicStrings, "cecp-my manifest", MapiPropertyType.String);
 var props = new PropertySet(BasePropertySet.FirstClassProperties, ItemSchema.MimeContent, extendedPropertyDefinition);'

Answer 1

Your property definition should be

'<t:ExtendedFieldURI DistinguishedPropertySetId="PublicStrings" PropertyName="cecp-my manifest" PropertyType="String"/>' 

That will return all the custom properties (as a Json String) which isSync should be in if set.