There is no means to send a star with a message - it's not part of the message sending protocol.

Didn't solve my problem, but no need to pursue further, if the program doesn't have this feature.

In a Write window under Options is a Priority selection. This is no guarantee that your recipient will see anything indication this status. Personally I have the Priority column turned off and I do not know how other clients handle this.

The star in Thunderbird has nothing to do with priority. It is just a reminder to you.

I just reread your posting and posting title. Here's a bit more information.

What you referred to as "stat" is also known as "flag" in the imap world. It does not affect the sending of the message.

Your title states "how to make a high priority message". In Thunderbird compose window pick options, priority, and you can see the available choices