Select the Thread Mill (Mill Cycles menu) command.
On the Lead tab, define how the tool approaches and leaves the thread by specifying the Lead In and Lead Out parameters.
On the Thread tab, ensure that Number of Starts is set to 1.
Threading starts at the position specified in the Start Angle parameter (which defaults to 0°), milling a thread of the specified Lead (Lead of the thread = Pitch of thread x Number of starts) and Depth of Thread.
Click on OK once you have defined the cycle parameters.
Select the arc geometry to use for the cycle.
In this example an external thread has been generated with a Start Angle of 270°: