Print with ABS or PETG with brim and without support. Cut bowden properly at 90 degrees and insert fully (15mm, using a rag and some force; the bowden inputs are slightly conical). It will hold very well. If you have problems, you could reprint the splitter slightly smaller, say at 98% or so.
Make sure to disconnect or switch off the AMS before feeding the external filament. A filament runout sensor switch at the external bowden input cutting the power to the AMS can do this automatically (see separate model https://www.printables.com/de/model/262945-bambu-lab-automatic-ams-switch).
You must use “UNLOAD” when removing the external filament, not manual retract. This will give the filament a clean cut and it will not get stuck in the splitter.
Please see also: https://www.printables.com/de/model/300273-bowden-tube-y-splitter-with-pc4-m6-connections and https://www.printables.com/de/model/584189-clean-bowden-y-splitter
The author marked this model as their own original creation.