X++ code on form level

1. Main form: InteleSmart 2 Receiver Form

2. On that form, there are lookup/drop-down field called Mag_ChannelNum and Mag_Frequency

3. Goal is to have options to select values. For example, if Frequency is 419 then Mag_ChannelNum should only display value of 1 to 32. (b/c there are long list of channel number)

4. How do I write a method?

Thanks for your help!