Open the game menu in one of the tabs of the bottom menu. Open "profile", then "Alarm and Response Regulations". Click on the button labeled " Create new Alarm and Response Regulation".
There configure your ARR with the following functionalities:
  1. Name: The name will be the one you see in the dispatch menu.
  2. Color: The Color of the button in the dispatch menu.
  3. Column number: The possibility of sorting entries in the dispatch menu screen.
  4. Category: New tabs.
  5. Stations: Will only use vehicles of the following stations.
  6. Hotkey: Which key to press to activate.
  7. Reset the previous selection: If this ARR is clicked, the previously selected vehicles will be removed from the selection.
  8. In these fields you can specify the vehicles and the amount of them to be included in this ARR
In this example the name for the ARR is determined by its composition: 2 FE = 2 fire engines (which are generic fire vehicles), + 2 ALS ambulances (advanced life support ambulance) and 1 K-9 (a K-9 unit, which uses dogs to solve crimes ;)). It has been given a blueish colour, so it can distinguished from other ARRs and put it into column two. It is a pretty general template so it can be placed into "General" as its category. Since, when this template is used I only want to send these units most of the time, I check the reset button. Then you just have to save it!
Et voilà: The ARR appears in the dispatch menu! If I click it now, 2 fire engines, 2 ALS vehicles and 1 K-9 units are going to be selected, if they are available.