If entering from a country with low infection rates, electronic registration is not necessary if you have proof in terms of the 3G rule (i.e. you have been tested, recovered or vaccinated).
Countries with low infection rates (no quarantine): Andorra, Australia, Belgium, Bulgaria, Denmark, Germany, Estlonia, Finland, France, Liechtenstein, Greece, Ireland, Iceland, Israel, Italy, Croatia, Latvia, Lithuania, Luxembourg, Malta, Monaco, Netherlands, New Zealand, Norway, Poland, Portugal, Romania, San Marino, Singapore, Slovakia, Slovenia, Spain, Sweden, Switzerland, South Korea, Czech Republic, Hungary, Vatican, Cyprus;
If you do not have a 3G certificate or if you come from a risk state or virus variant area, electronic registration is mandatory before entering Austria, but must not take place earlier than 72h before entry.
You will find the electronic PTC registration form in English here.
You can download the completed form or receive it by email and present it to the authorities electronically or as a printout if checked.