bamshad I only know of https://distrotest.net/ (free) which you can try a full range of different Linux OS from the web.
For Windows I guess virtualization is the way to go, either on your own machine or in the cloud at for instance Azure.
It’s worth pointing out that you get 12 months free services at Azure and $200US credits to run Azure services for.
If you virtualize on your machine I would recommend VirtualBox or VMware workstation.
You can also utilize the Azure DevTest Labs, this is a test platform where you can ship and test your software on Azure VMs. Beware that this can drain your Azure credits quite rapidly.
But I assume running your application via Azure DevTest Labs is a bit like this:
But its very nice of you to share your application with us 😁
Looked very nice when I browsed through the documentation!