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The issue you describe must be a glitch from your side only as I try integrating my form with Dropbox and Box and I had no issues.
Could you please provide further information about the issue you are getting at the time of building the integration? Does it gives you any error message?
It is possible that the cause is related to the browser you are using and that's why you cannot complete the integration, we recommend to use Chrome browser to create yoru forms.
We will wait for your reply to give you further assistance.