All you need to do is to export iMovie to .MP4 video file and upload to OptiSigns.
To export iMovie to MP4:
Step 1. Launch the QuickTime and Get into the Main Interface
Open the included QuickTime software on your Mac and click the button of "Share" in the toolbar, then select "Export using QuickTime".
Step 2. Export the MP4 Video Format
In the pop-up window "Save exported file as...", after entitling the exported video name and navigate to where you want to save the file, click on "Export" option and you will find "Movie to MPEG-4" in the drop-down menu. You can also make some settings if you need. Then hit the "Save" button and a "Share Successful" notification will appear. So your iMovie has been exported to MP4 successfully.