I would use something like this:
CREATE EVENT SESSION [truncate] ON SERVER
ADD EVENT sqlserver.sp_statement_starting(SET collect_statement=(1)
WHERE ([sqlserver].[like_i_sql_unicode_string]([statement],N'truncate table%'))),
ADD EVENT sqlserver.sql_statement_starting(SET collect_statement=(1)
WHERE ([sqlserver].[like_i_sql_unicode_string]([statement],N'truncate table%')))