Which one should I use if I want to describe the field where theory is put into application?
I hope I present my question clearly. Applied mathematics is such a field. But I don't know how to call it appropriately.
You do not want to specify one particular field, and anyway there is no specific applied field related to pure mathematics (you don't want to write applied mathematics), so you should use the indefinite article - and applied: ... plan to study in an applied field, such as computer science or engineering.