Your ignition switch, or more correctly, your starter switch (the electrical part of the lock) is shot and needs replacing.
Pretty simple procedure, documented here - do a search. There is a switch available from Porsche, an Audi Switch (the one you pull out will have an Audi symbol on it), and a generic mayle switch (probably the mfr. of them all). You can find them on ebay and a number of porsche parts suppliers. More difficult to source from some dealers as Porsche wants you to replace the entire mechanism (mechanical and electric) which is much more involved and $.
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