Hi Stewart,
When no input is provided, the result should be different than pressing Cancel - empty string for no input, null for cancel.
At the moment, there is a bug in that, you can Follow it here: https://feedback.telerik.com/blazor/1536274-differentiate-cancel-and-ok-with-an-empty-textbox and I am marking this thread as a duplicate to that, because the core of the issue is telling the two user actions from one another.
If you need to have a popup that has a mandatory field, I recommend you wrap the TelerikWindow component in a component to provide the desired UX there, with the desired inputs, validation, messages and so on.
Regards,
Marin Bratanov
Progress Telerik