@RowenaT From my experience, the Subject (Line) or any other parameter you are trying to use in the definition has to occur at least once before it’s available to be selectable from the dropdown.

One workaround I’ve found is to use the “Message” condition instead of the “Subject” condition.  And in the place of the Message value (Campaign Name or Flow Name), you can actually put the Message Identifier (Message ID for short) instead.

See this:

The Message ID of any message (Campaign or Flow), is the alphanumeric code in the URL of the message.  For example, a message from a Flow, if you clicked through it, may look like this:

https://www.klaviyo.com/flow/message/Q94Ujx/content

The value Q94Ujx in this URL here is the Message ID in Klaviyo’s system.  And using this seems to work even before the message has been sent. 

Also, using “Message” condition instead of the “Subject” condition has the benefit that if you were to change the Subject Line for any reason for that same message, it still matches without changing the condition definition (unless you’re specifically targeting an exact Subject Line for some reason).  

Hope this tip helps!

You are an absolute legend!  This worked great!  Thanks so much.  Really appreciate it!