You can get a USB-C PD to barrel jack adapter that ought to do the trick. I use one with most of my fleet on a regular basis (but not my L14). Most systems (not speaking about Purism specifically), have a tolerance of at least +/- 10%, so even if the PD adapter feeds it 20V you’ll probably be OK. YMMV.
Why do this instead of PD directly? Based on some of the other users’ experiences, it sounds like it can be difficult to charge an L14 from flat via USB-C.