Does your under hood tank have a drain that you could install a nipple in to in order to add the pressure? You shouldn't need more than a few psi to hopefully find the leak.
Harbor Freight's kit (SKU 63862 or 63893) has three universal adapters on the right side of the pic below. If needed, I can take a picture of how to set it up on the test tube.There are coolant system pressure tester tools, hopefully one would have a fitting for the HX cap?
And make sure you stay below 5 psi as stated on the test tube lid.Do not overpressurize the HX look unless you know the brick won’t be the piece that blows a leak.