The oldest submission I can find using the search form is this one from the 8th of March 2006:
https://hwbot.org/submission/498730_trouffman_cpu_frequency_pentium_4_561_5263_mhz
It's hard to know whether this is really the first sub as the fact that past world records were added "posthumously" clouds the picture. HWBot's copyright is 2004 - present (as indicated by the page footer), does that mean the project was in startup mode 2004 - 2006 with no submissions made or have those subs made in that timeframe been purged somehow?