I want to export to Compressor using proxies.

If you export using Compressor settings, instead of sending to Compressor, it will use the proxies.

Go to Preferences->Destinations, drag "Compressor Settings to the list on the left, rename appropriately - e.g., "HEVC using Compressor", and use that destination when exporting.

If quality isn't an issue why don't you just export just export QuickTime or MPEG-4 from FCP? What's the point of going to Compressor?

You only need to add one Compressor setting into the FCP X>Share Menu, you can add more if you have need of regular settings.

Once you Share/Export in FCP X you can CHANGE the Compressor settings to another you have already created:

Al

Or you can have multiple FCP share settings with different Compressor presets that you can access directly in FCP. Regardless of the way you do it, Compressor settings or FCP built-in presets, the encoding engine is the same.

Because Compressor is lightning fast, and better quality.

Any custom setting you can create in Compressor is available to use as a setting in FCP.

I think that’s the best solution. It’s annoying that when you try to output H.264 by way of “share” in FCPX, you can’t tweak any settings, other than to pick Vimeo or YouTube, etc.