I believe that a corpus distribution should be reported on 1041 schedule B line 10 but I can't figure out how to make TT enter that value without doing an income distribution and schedule K-1. I want to keep the income in the trust and not distribute current income to the beneficiary.
"How do I make TT report a corpus distribution from a complex trust without an income distribution?"
It is not technically correct, absent the circumstances set forth in the instructions for Form 1041 (link below), but with TurboTax Business you can make several overrides in Forms Mode on Schedule B. Obviously, you want Line 15 of Schedule B to be blank (or otherwise show a zero).
You should be aware, however, that distributions, whether mandatory or discretionary, carry taxable income through to the beneficiary(ies) to the extent of DNI.