You can use the ShowDialog command. Make sure when you inject your form you select "Rich Client" in the compatibility section.
Sample command syntax:
ShowDialog /text=Click here to continue. /buttons=OKCancel
The QdabraRules result node will have the DialogResult - so, in the case of OKCancel the result field will be either OK or Cancel.
So, your logic could be - set Command to the ShowDialog command, then the next rule can check the Result - if it OK, run the rest of your rules that would complete your form submit (or whatever it is you wish to do). The command also has optional icon and caption parameters.