I know this is an old post, but figured I would respond if anybody still needs a solution.
First of all the behavior described by LemonNoIce is still the same (as tested in my 2023 long range Y in Feb 2023). But after a little head scratching I found an easy workaround if you want to setup a trip with custom superchargers (for example if you plan out a trip in ABRP):
1) In ABRP, plan out the trip, then click Share, then export to Excel
2) Make sure trip planner is turned off in Tesla navigation settings (as you dont want Tesla inserting charging waypoints automatically)
3) If another route is in progress tap "End Trip" to reset navigation
4) Voice command: "Navigate to XXXX" where XXXX is your final destination - now your route is in the system without any supercharger waypoints.
5) In reverse order as listed on the Excel sheet, add each supercharger as follows:
Voice command: "Baker California Supercharger"
If the "Add" dialog box comes up immediately after the voice command, then tap "Add".
If multiple options appear, DO NOT SELECT FROM THE LIST, instead, select the pin on the map corresponding to the desired supercharger, then tap "Add" (this will avoid resetting the trip with this supercharger as final destination).
6) In case the superchargers were entered in the wrong order, tap "Edit Trip" and reorder by dragging the "hamburger" icons shown at the left of each waypoint.
You should now be have your trip ready to go with the custom supercharger waypoints, and without the need to do any typing on your Tesla screen.
I hope this helps.