The PRO-93 is the same as the PRO-95 but with less memory/channel capacity. As far as I know there is no way to wait for a trunked channel. As you stated, you must wait for a transmission, then press and hold TRUNK until the scanner beeps. I've also found that the "hold" above is only good until the control channel is switched to a different frequency (the scanner will not search for a new control channel in that mode).
However, on the PRO-97 there is a way to do what you want. Navigate to the correct bank. Press FUNC TRUNK. Navigate to the correct TalkGroup ID. Then press FUNC TRUNK again. But here again I've found bugs in this and apparently you must first scan long enough for the scanner to have found the control channel in the bank BEFORE performing the above.