I have booked a flight to the UK transiting through Vienna International Airport, and recently I received an email (from the airline) asking me to fill out pre-travel-clearance for Austria, which includes address of quarantine (Austria), but I'm not staying in or entering Austria, so I'm not sure if I need to fill out this form.
This is the form website https://entry.ptc.gv.at/en.html
Press "One or more of the exceptions mentioned in § 7(1) or § 8 of the COVID-19 entry regulation are applicable to my entry." and it'll say you're exempt. Furthermore, the site then won't even let you press the button to submit (it also requires an Austrian address, which is obviously not applicable).
At the airport, follow the signs for the D/G gates. You may face a military checkpoint. If so, show your passport/ID and say you're transiting to the UK, and show your ticket if asked.
According to the Pre-Travel Clearance website of the Austrian government, you are exempt from filling out the form.
In accordance with Section 7 para. 1 and Section 8 of the COVID-19-Einreiseverordnung registration with the pre-travel clearance form is not required: […]
When entry is related to other exceptions in accordance with Section 8 para. 1 and Section 8 para. 2: […]
In the case of transit passengers (they must also leave the country promptly)
On arrival in Vienna, you might be required to show evidence for your (immediate) continuing flight out of VIE.