Well, timing something with no indicator means that you yourself have to be the indicator. So in order to time it, you have to emote or say something into a chat channel. Then you make a custom trigger based off of what you did. Assuming that TS will affect what ever you are timing, you must include the name of the mob you are trying to time into the text you output.
For example you can say something like, "Venril Sathir has cast mez" into some channel. Your custom trigger regex can be
PHP Code:
[^"]+"(?<attacker>Venril Sathir) has cast mez"
The attacker part tells ACT that it should check if that combatant is under the effects of TS before creating a timer.
Telling it to create a timer is just checking that checkbox and deciding what to name it. Once you name it, create a normal Spell Timer with that name.