Hi santoshslonkar,
Thanks for contacting Syncfusion forums.
Kindly enable the property “AutoPostBackOnItemCheckedChanged/AutoPostBackOnTextChanged”
to invoke the post on each selection in MultiSelectionDropDown control.
You may also refer the below given
documentation link to know more details regarding this requirement,
http://help.syncfusion.com/UG/User%20Interface/ASP.NET/Tools/documents/51831serversideevents.htm
Let us know if it helps.
Regards,
Varalakshmi