Hello and welcome to InfoPath,
To automatically populate a certain field on an InfoPath form, you will need to create a rule which will set the target field's value (box 2) if the initial field (box 1) reaches a certain condition. So let's say you have Drop-down values: A, B, and C. You will need to create rules for each of these values.
Example:
Rule 1: Set box 2 to a value of 1; on the condition that box 1 is equal to A
Rule 2: Set box 2 to a value of 2; on the condition that box 1 is equal to B
Rule 3: Set box 2 to a value of 3; on the condition that box 1 is equal to C
It should look similar to this:

I am also attaching a sample form for you to test. You'll be able to see these rules if you click on the box1 field. Hope this helps.