Should be okay as a one off situation.
Check on the cable rating, it is printed on the outside sheathing.
It needs to be no less than a cross rating section of 1.5mm.
Larger ( 2.5mm ) is fine, but not any smaller than 1.5mm.
Some of the cheap windup ones are only rated at 0.75 / 1.00mm - NO GOOD !.
This size flex will over heat and is dangerous.
Okay for your Xmas lights etc but that’s it.
If the cable is in good external condition and it gets a little wet, there is little risk here.
It’s the socket outlet that your Granny lead plugs into, that is the vulnerable part that needs to be protected from the rain.
Try and keep the socket outlet off the ground and as a temporary measure, do you have a large plastic storage container that could be used to put over the socket outlet to shield it from the rain

️ ?.
If you intend to use a socket outlet externally, then it should be suitably IP 55 / 65 rated.
This rating covers both dust and water ingress.
As other members have said, monitor the heat being produced at the three pin plug that you are using to power your extension.
The plug top WILL get warm to the touch, which is completely normal, but if it gets mad hot ? to touch, then stop.
The plug on the charger is unlikely to get hot because it carrying a heavy duty plug and cable.