You can request a specific sub, for any request, as long as they are linked to your school. 

You will do so by selecting the amount of time you want to give them to accept the request and then click on the 'Choose Subs' button. If the request is less than 48 hours from the start time, you will only be allowed to give them up to 12 hours. 

Once you click 'Choose Subs' you will see a list of your favorite subs, and all subs who are linked to your school. You can choose any sub who is not greyed out; With this example, Candice would be your only choice. Why can't you choose a sub who is greyed out? See our help article for more information.