One way would be to make a simple class-A transistor amplifier circuit. You would feed your 1.5 MHz CW (continuous wave... "always on") signal into the base of the transistor amp. The supply voltage (on the drain/collector) would be supplied by your pulse-generator. When the pulse generator goes high, your amplifier has power, and lets the 1.5 Mhz signal through. When the pulse gen goes low, the amplifier has no power, and the 1.5 MHz signal doesn't pass through.
This assumes you can get something on the order of a few volts out of the pulse generator, and that it can source a decent bit of current (a few tens of mA's). This is form drain modulation (if you use a FET, since they have drains).
Another way to accomplish this task might be to use a transistor as a shorting switch. In that case, the 1.5 MHz source goes through a small series resistor (you'll see why later), to the output of your system. Connect a transistor from the output node to ground (1.5 MHz signal is on the drain/collector, source/emitter is grounded). The base/gate of the transistor would be driven by your pulse generator. When the pulse-gen output is low (0V), the shorting transistor is "off" and the 1.5 MHz signal passes right down the wire to the output. When the pulse-gen output goes high, the shorting transistor is turned "on", and the output wire is pulled to ground (0V). The 1.5 MHz oscillator will then "see" the series resistor as it's load (the other side of the resistor is connected to ground by the shorting "switch"). By giving the oscillator SOME load, you'll have a chance of keeping it stable while you toggle between opening and shorting the output node (oscillators don't typically like zero ohm loads, i.e short circuit).