The command framework in Fusion doesn't support the idea of applying the current state and continuing to run the command. Inventor supports this for several of its commands but you won't see this for any Fusion commands.
I also wanted behavior like this for a command I wrote that creates a construction point at a defined coordinate. I didn't want to have to invoke the command again for each point so I added a "Create" button on the dialog, which is the equivalent of "Apply". However, it isn't really creating the point when I click "Apply" because the creation of persistent data only happens in the execute, which his only called when the user clicks "OK". Instead, when the user clicks the "Apply" button I add that coordinate to a list and draw custom graphics to show a preview of where that point is. The user can now define another coordinate and click "Apply" and that point will be added to the list so a preview is shown. If they click "Cancel" everything is gone. If they click "OK", then I create a real construction point at each coordinate.

I didn't have to use custom graphics for the preview but could have created real construction points but they are heavier to create and they would get aborted with every change to the input and I would have to re-create them. In my case, custom graphics were also more visible to make it easier to see what they had defined so far.
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Brian EkinsInventor and Fusion 360 API Expert
Website/Blog:
https://EkinsSolutions.com