I don't have an Indigo but how I sync some of my other synths is:
In Logic go to Settings>Synchronisation>MIDI then check the transmit MIDI clock button & select the synth/port you require
In the synth settings find MIDI clock settings then set to External, once you have done this you will need to press play on Logic to start transmitting MIDI clock to the synth otherwise when you press a key you will hear nothing & think it hasn't worked.
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