Change the shipping method depending on the value of the additional field

there is no such field, but you can use 2 actions
1. Stop execution
2. Fill in the process or contact field
The logic is the following:
In the "Stop" action, specify the condition that must be fulfilled (IMPORTANT read the name of the action, if you need the system to do something further if the condition is met, then the condition should sound like "not equal", since the action itself, on the contrary, stops doing something if condition met)
and the second action put "fill in the process or contact field" it allows you to write the name of the delivery method in the standard field, the main thing is that the name matches 100%