There is no resetting it. If the THEFT light is blinking, there is some type of problem with the system when it tries to read the key and authorize startup.

When the theft light blinks like that, there will be stored trouble codes that identify the details of the problem, and then the appropriate diagnosis can be done. One common fault is the transceiver. It is a piece that surrounds the ignition cylinder where the key inserts. The transceiver is responsible for reading the key. It has wires that go down the steering column, and a connector near the base of the column. That is one possible failure among many. The key itself could be bad, it has a transponder chip in the head, and must be programmed into the system properly. If this is a new key, the key may be the problem.

Unfortunately, fixing this problem may mean a trip to the shop, they will need to use a Ford-compatible scantool to find the codes and then they will know what to do next.