2 ways,
1: set all your pre tanks as "primary tanks" - right click on them -> SET AS PRIMARY
Tank leader will select your primary tanks first (if they are all "inbattle" then it'll select the other ones)
2: open TankLeaderSettings.xml in notepad or any text editor..
Edit the
<QueuePriority>
<Entry></Entry>
</QueuePriority>
Put the NAME of the tank you want between <Entry>NAME OF TANK HERE</Entry>
Make a new Entry if you have more tanks you would like to use.