ZERØ for Citrix application is specifically designed for MDM deployment via Citrix. This app allows both ZERØ and Citrix customers to take advantage of a secure Intune MDM deployment, organize and protect BYOD environments with mobile application management (MAM).
If you do not intend to use ZERØ with Citrix MDM then you can download the main ZERØ application here: https://apps.apple.com/us/app/zero-email-for-attorneys/id1216474378