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There is a little trick you can try. Not strictly a poster frame, but at least in a Mac will do the job.

1) Open the exported file in Quicktime Player.

2) Move the playhead to the frame you want to use.

3) Press Command-C to Copy.

4) In the Finder, select the file and press Command-I to open the Info window

5) Select the icon at the top, and press Command-V to Paste.

Not with Final Cut Pro. You can choose a poster frame in Compressor. Compressor is a separate program, which is fine if you already have it; but I don’t think that this feature alone would justify paying $50.

Hi

Right click on the Clip and go to "Get Info"

Drag any jpg image onto the icon in the top right corner next to the clip title

and thats all you do

The poster frame is taken from the first few seconds of the video. Whatever’s in that area is likely to be selected.