The wires go thru the battery case of the Beats and the MIDI-Out-is housed in an old MIDI-Thru-Box (MT-5). This item routes in normal life 3 MIDI-Ins to 5 MIDI-Outs, but now the 5th Midi-Out is the Output of the Volca Beats!
If I connect the MIDI-Out of the Volca Beats with one of the MIDI-Ins, I have 4 Midi-Outs to sync to the Volca Beats. At the moment I only use it for the Boomerang III Looper, but in the future I will try it with my other Synths & Sequencers (Jomox Xbase 09, Analogue Solutions Europa, Roland Juno 60 with Midi, Waldorf Pulse I & Microwave I , Doepfer MS-404, Nord Modular Rack ....

Thanks to Marcbert for Soldering The MIDI-Out!