To activate Screencastify Unlimited access, follow these instructions:
Visit this page and click "Redeem coupon" (under the Next Billing Date)
Enter code CAST_COVID
Click "Activate License" (no credit card required). Ignore the Next Billing Date - that will be incorrect
Screencastify's website states that this offer is "at least until April 30". I interpret that to mean that there's a possibility that it could be extended.