Purpose-built flasher, as suggested above, is best bet.
Otherwise, trying to apply Mr. Ohm's and Mr. Kirchoff's laws, looks like something around 10-12 ohms might work . . . loose quantification.
Rationale; no flaw-free guarantee:
Stock, we have 23 watts. VI = 23. But V = 12. So, I = 23/12; about 2 amps current.
With the trick LEDs, VI = 10. V ain't changed; still 12. I = 12/10; or about 1 amp.
We want the flasher to see TWICE that, so it will flash. With the LED in the circuit, IR = 12. Let I = 1, R = 12 ohms. We want to halve the resistance, double the current, so . . . let's wire in a parallel 12 ohm resistor to increase the current and the wattage to trigger the flasher.
What? You tried that, and it didn't work?
Sorry! I said a purpose-built flasher was preferred . . .