Launching a ThinApp from Horizon Shows a Prompt for the First Time:
Do you or your users get an error "Do you want to allow this website to open a program on your computer?" prompt the first time they go to launch a ThinApp package in Horizon?
In most cases, the prompt is not a big deal for Horizon users on a Windows system where ThinApp packages are being deployed. But if you have a dynamic desktop environment where users are dynamically assigned a virtual system or randomly use physical desktops, this could be an issue.
First, it's important to note, this is not a Horizon or ThinApp specific issue. Rather it's a security configuration within Windows.
Here is how to resolve this issue. Simply create a GPO with a custom registry value or manually edit the registry of your virtual desktop master gold image to have the following registry setting within.
Create a DWORD called "WarnOnOpen" and set the value to “00000000”.
The MSDN Blog article "URL Protocols, Application Protocols, and Asynchronous Pluggable Protocols. Oh My!" explains this behavior and the fix under the Application Protocol section within Windows, which is basically what Horizon is using when it launches a ThinApp package.