Then choose a name that is somewhat related to the unit, something you can remember late.

The purple text is the player/merc speaking:

You need to have a greetings and a goodbye, the goodbye is not mandatory but let's keep it for now because it is easier to learn this way.
You can put a text that will be the first thing the actor says after being clicked.
The dialogs in JA3 come with similar context to JA2, where there were only "DIRECT", "RECRUIT", etc.
This is not really a phrase but it is like the intention of the question.

But you can add a new phrase and then let the "Player" talk like this:

You can follow up the conversation of the actor:

Or you can add an interjection of a second actor:

Now, all conversations need a "go to -> end conversation"
You can set this to any part of the dialog, including in the greetings and skip the goodbye
but this way is easier to learn when you have all pieces separated.

You need to choose an effect:

then pick unitstartcon

pick the convo

If don't show you can just type it.

Now you can save and test: