Somewhere in the far past I had that problem and I called for help from Google on this and I discovered that the keys OPTION-COMMAND-ENTER works to enter a line break. Muscle memory ever since has show it works all the time. I might be mis-remembering, but I recall CTRL-ENTER used to work, but now the OPTION-COMMAND-ENTER works. I think something changed in macOS along the way.
Ah … now remember this from June 2025 discussed here (found from search):