Download the MSIX Package of the MessengerApp and follow the instruction below to install the app certificates and then install the MessengerAPP.
1. Right click on the MSIX package, click on Properties and then go to "Digital Signature" tab. Select the certificate from "Signature list" and then click "Details"
2. Click "View certificate"
3. Click "Install certificate"
4. Select "Local Machine" and then click "Next"
5. Select "Place all certificates in the following store" and then click "Browse"
6. Browse to "Trusted people" and then click "OK"
7. Click "Next"
8. Click "Finish"
9. Click "OK"
The required certificate is imported now and you can begin installing your MSIX package.