Hi there,
So the problem would be the Charger in the Xiao if you connect the External charger to the Batt terminals and then connect a USB-C it will try to charge the external charger. (problem) Some can act as a Battery eliminator and have the Diodes already. YMMV
I know takes a few more posts maybe , but you will get there… 
I have never done it that way, so you’ll be on the bleeding edge, exspect some cuts 
I have seen a Solar Power Brick (internal Battery) connected to the
USB of the Xiao and that worked pretty well. Cloudy days the Battery kept it going no problem, Normally it’s just on and recharged by 10 am (south florida)
I 'm a purist in that way , I either use a battery connected to Xiao and it handles the charge management or USB-C power brick… Always works. No diodes, no Shenanigans 
HTH
GL 
PJ 