There is no official support for USB-C charging on this system.  However, trickle charge with USB-C while the system is off should be possible.  Use the thunderbolt port as it's wired for power delivery.

Since it's a non-support method, there is no guideline for the maximum power requirement.  You can attempt with 130w USB-C charger, preferably Dell brand charger from XPS and Precision product line.

Thanks Chino, i'm just a bit afraid that it would damage the motherboard or battery with 100w plus attempt so want to double check before trying. the usb c thunderbolt port is set up for PD charging external devices but it looks like that's below 30w output? would it suggest the same for input limit?

@Chino de Oro​ that's very insightful thank you Chino, i'll give it a go =)

I can see in the manual for this system that it can do up to 15 W power share through the thunderbolt port, but I assume that means output, so to charge a phone or whatnot. Is there any documentation that officially states what it can handle for input? I have a nice 100W Anker USB C charger that would be lovely to use instead of the big brick for evening work from home tasks or quick backup power during meetings. I also have a Dell WD19 dock in my office that would be nice to have as backup input in case I forget my charger or something. 

@Chino de Oro​ Hi, Chino, I had buy an m16 r2 alienware few days ago. Now I search a dock for it. I see the WD22TB4. I'm asking you if you were sure that the wd22tb4 works on this model. If it's true, I'll buy it today :)

Many thanks

Regards

Officially, Dell have not tested or validated any docking stations for Alienware laptops.  But, verifying with Dell Commercial Docking Compatibility for similar builds and high power demands systems such as the Precision line, you will find that most of high power Precisions are compatible with WD22TB4 dock.  Note that while you can take advantage of the latest thunderbolt 4 bandwidth, the power delivery still has its limitation.  Your m16 R2 power adapter must be used while docking for optimal operation. 

@Chino de Oro​ Sorry, I don't know if I understood correctly, so are you telling me that, while I'm using the docking, I still have to connect the original power supply? The WD22TB4 doesn't have a power supply? 

That's correct, the docking station with power adapter can only provide up to 130w of power delivery.  Your Alienware needs up to 280w of power for optimal performance.  Therefore, it can not run heavy workload using power from the dock.