As far as I understand you want to execute some custom logic ( or prevent the appointment insert for example) based on the selected resources for a specific appointment. In the code snippet below you can see how to retrieve the appointment resource values. Once you have access to those values you can add an attribute to the appointment for example or execute some other custom logic or prevent the insert command.
sender, AppointmentInsertEventArgs e)
ResourceCollection resources = e.Appointment.Resources;
//here goes the logic based on the resource values
If you want to modify the controls within the advanced form before the advanced form is shown to the user you can review the following help article
. You are able to do that on the client-side as well as shown here
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