With all the customization options in powerapps, it seems as though a cascading dropdown in a model-driven app form should be easy.
Looking for a way to filter regions - the user selects a country, and then the regions dropdown only shows regions in that country (If the user selects USA, regions dropdown shows US states, if the user selects Canada, regions dropdown shows Canadian provinces)
Do you want to configure a cascading dropdown in a model-driven app form?
If you want to configure a cascading dropdown in a model-driven app form, I afraid that there is no way to achieve your needs in PowerApps currently. Cascading Dropdown functionality is not supported within Model-Driven app form.
If you would like this feature to be added in PowerApps, please consider submit an idea to PowerApps Ideas Forum:
As an alternative solution, you could consider embed an canvas app within your Model-driven app form. Then use the embedded canvas app to add data to your CDS Entity. Within the embedded canvas app, you could configure a cascading dropdown functionality between the Region Dropdown box and the Country Dropdown box.
More details about embedding an canvas app into a Model-Driven app form, please check the following blog: