I have a device TRX-2 does not give this option.
So for the TRX1 there is an easier way (I assume the TRX-2 is similar?):
Go to Search > Limit Search and enter the upper and lower frequencies. Once searching then press Menu and enable the mode(s) you wish to detect in search - eg tick NXDN. This is very helpful albeit you can only search one band at a time.
With a Uniden you can set up a custom search or multiple custom searches, but you cannot enable it to decode just a single mode like the Whistler. Both Whistler and Uniden could benefit from adding these respective features to their scanners.