Repository URL: https://releases.threema.ch/fdroid/repo
If you're reading this on your phone, click here.
Otherwise, scan this QR-Code with your phone and open it with F-Droid:
If this all didn't work, open the F-Droid app, open Settings > Repositories,
tap the "+" button and enter https://releases.threema.ch/fdroid/repo/ in the
Repository address field.
If you would like to manually verify the fingerprint (SHA-256) of the repository signing key, here it is:
57 34 E7 53 89 9B 25 77 5D 90 FE 85 36 2A 49 86 6E 05 AC 4F 83 C0 5B EF 5A 92 88 0D 29 10 63 9E