Hi Supriya,
in your screenshot I can see the request header Content-Type = ...;type=feed; ...
If I retrieve an entry, I receive a response with Content-Type = ...;type=entry;...
You may try it again with type=entry. Even easier, you can shorten the content type and work only with Content-Type=application/atom+xml and nothing more - for me that works.
The quickest way is to do a request beforehand, where you retrieve a single entity, e..g. when entity with ID 22222 already exists, then you do a GET for ZREALTY_SRV/ActivityCreateSet('22222'). Then you have the chance to take exactly the response body, copy it into the request body, change the field values accoding to your needs, change the HTTP verb to POST and submit the request again.
Lots of success,
Ringo