This mark on Alif (which is small صـ by the way) is not essential in the modern standard and found mostly in Quran calligraphy, to bring attention on how reading should be. In normal writing, we don't use it really. We just write ا without Hamza ء and that's called Hamzat-Wassl همزة وصل.
On keyboard, I don't recall it's possible to add it without a special aid from a third party, like for example "inserting symbol" in Word or using the "Character Map" that comes with Windows. In regular keyboard, I don't recall it exists anywhere; Not in my mine at least. This is also the case with Dagger-Alif (or Alif Khanjariyyah) which can be found on هذا.
If Duolingo considers an answer wrong because this mark is not typed, this is a big and ridiculous mistake (just as is the case with Dagger-Alif). They should adjust these conditions because, well, they are "abnormal". We don't use these marks in official letters or in daily writings or newspapers.