A DDL trigger helps you to monitor when a table is Created , altered or dropped. You can perform some auditing whenever these happen.
You can create a database scoped DDL trigger as shown below
create trigger ddl_trigger on database
'access denied to alter/drop this table'
But you can not see the script of DDL trigger using SP_HELPTEXT like other DML triggers
When you execute the below
EXEC sp_helptext ddl_trigger
You get the following error
Msg 15009, Level 16, State 1, Procedure sp_helptext, Line 54 The object 'ddl_trigger' does not exist in database 'test' or is invalid for this operation.
The other way is to make use of sys.triggers catalog view to identify and script out the definition using Object_definition function
select object_definition(object_id) as definition from sys.triggers