You can open the Dropbox export dialog via one of the following methods:

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 Link to Dropbox..., this will open the Dropbox Authorization dialog.

Once the Authorization dialog appears, login to your Dropbox Account, and follow the prompts to allow the application to access your online profile.

After authorization is complete, you can click the Create button to commence the export and upload process.