You would need to customize the workflow code to retrieve the name of the requirement linked to the test, and plug the value into you new field. Customizing workflow code is described in the Administrator Guide.
You will probably also have to get acquainted with the Open Test Architecture API, and you will need to use some of that functionality in your code.
And you need to decide what to do if the test is linked to more than one requirement.
You will probably also have to get acquainted with the Open Test Architecture API, and you will need to use some of that functionality in your code.
And you need to decide what to do if the test is linked to more than one requirement.