You don't need UO Assist for this.
Make a macro in the Classic Client that has four lines to it, like this:
Line 1: Say: Trick or Treat
Line 2: WaitForTarget
Line 3: SelectNearest: Mobile
Line 4: CurrentTarget
If you want to make a macro to use the begging skill, just substitute the following in Line 1: Use Skill: Begging .
Also, and very important for both types of macros, make sure that you don't have a check mark in front of the option to Disable New Targeting System.