Convert 24 V to 20 mA (none is variable)
Connect the 4-20 mA input to the 24 V supply with around a 2 k&Omega; &frac12; W resistor in series.  Even if the 4-20 mA input drops 10 V (unlikely), then it will still be driven with (14 V)/(2 k&Omega;) = 7 mA.  That's well above the 4 mA minimum that you should be able to detect it with maybe a 5 mA threshold.

The &frac12; W rating is in case the 4-20 mA input drops no voltage at all, or you accidentally have a short.  In that case, the power dissipation will be (24 V)2/(2 k&Omega;) = 290 mW.