Failed to fetch Microsoft Exchange free/busy data

When we configure user calendar to connect to Office365, we got below exception.

jvm 1 | 2019-09-02 15:47:03 GMT+08:00 (c.w.p.p.w.p.ExchangeCalendar:WARN) [qtp559619280-255] [RID:36] - No value found for email user attribute
jvm 1 | [Fatal Error] :1:1: Premature end of file.
jvm 1 | 2019-09-02 15:47:03 GMT+08:00 (c.w.p.p.w.p.ExchangeCalendar:INFO) [qtp559619280-255] [RID:36] - Failed to fetch Microsoft Exchange free/busy data, trying again, using EWS…
jvm 1 | 2019-09-02 15:47:03 GMT+08:00 (c.w.p.p.w.p.CalendarPrincipalAttributeProvider:WARN) [qtp559619280-255] [RID:36] - Error code: 403 - Forbidden
jvm 1 | java.io.IOException: Error code: 403 - Forbidden
jvm 1 | at com.webmethods.portal.portlet.wm_xt_pap_calendar.provider.ExchangeCalendar.fetchFreeBusyEWS(ExchangeCalendar.java:428)

and found a solution:
To enable cross-forest free/busy requests at the organization level, you have to add the User agent to the EWS Allow list.

But don’t know how to use this solutoin.