I have the same predicament. I’m not 100% certain but I think the barrel plug needed should be a size 4.5*3.0mm. These usually have a blue plastic tip at the end of the plug.
I tried the various barrel tips I had laying around but none fit…all had either too wide of outside diameter or too small of inside diameter for the Soundfreaq antenna jack.
There are many inexpensive kits available on Amazon, eBay, etc. if you want to pick up some to test.
Any copper wire will work as an antenna, possibly even without a plug attached.
You can also email Soundfreaq support at Soundfreaq.com to ask if they have a replacement antenna.