According to F1 help:
dBm(x,Z)
convert voltage across impedance Z into (active) power in dBm (parameter Z is optional, default 50)
dBm() should be applied to voltages. S-parameters are dimensionless voltage ratios.
S21 is 1 for such a circuit. An impedance of 50 Ohm and 1 V_peak would dissipate 1/100 W of power. Equals +10dBm