You can open the Dropbox export dialog via one of the following methods:
Click the Save Video drop down button. Select the Dropox option.
Click the Save Video... menu item and in the wizard that opens, select the Upload, then the Dropbox option.
Once reaching the Dropbox export dialog, you can choose assorted settings that will be applied to the exported project.
After making your selections click on Create, to being the export and upload process.
If required the Dropbox Authorization dialog will appear. Login to your Dropbox Account, and follow the prompts to allow the application to access your online profile.