Relate tickets to one another via REST API

Hello,

I need to create a relationship beetwen change and request (RequestedCausedByChange type). With documentation and this topic https://community.microfocus.com/it_ops_mgt/itsma_suite/f/itsma-q-and-a/326768/import-configuration-management-sacm-relationships-many2many  I was able to create a body for my post:

{"relationships":[{"name":"RequestCausedByChange","firstEndpoint":{"Request":"XXX"},"secondEndpoint":{"Change":"YYY"}}],"operation":"CREATE"}
 
where XXX is the Id of my request and YYY is the id of my change. I use EMS/Bulk API. I got error 500 though I do not see any reason why?

Do you have any ideas what to change in my body to get the relationship done?
Thanks!
Marcin Plisz