How can AM modulation be performed in LTspice by built in MODULATOR function?
As stated in the help:


  The MODULATE device is a voltage controlled oscillator. See the
  example schematic .\examples\Educational\PLL.asc. The instantaneous
  oscillation frequency is set by the voltage on the FM input. The
  conversion from voltage to frequency is linear and set by the two
  instance parameters, mark and space. Mark is the frequency when the FM
  input is at 1V and space is the frequency when the input is at 0V. The
  amplitude is set by the voltage on the AM input and defaults to 1V if
  that input is unused(connected to the MODULATE common).


So you place that component, connect the bottom port to GND, and write the frequencies you want into the Value part in the format of mark=xyz space=abc mark will be the frequency when 1 V is supplied to FM and space the one when 0 V is supplied.

At the AM port you just put some sine or whatever: