In the DEV forums Launchday FAQ there is a detailed explanation. But in short
Right-click Company of Heroes 3 in your Steam library and select Properties
Under Launch Options, input the following argument string without the quotes, and be sure to change “filename” to whatever you saved your replay file as. (So "temp" if you want to watch your last game.)
"-replay playback:filename.rec"
It will launch the game and then directly load the replay. Edit- it will only be from your perspective so limited usefulness if you are looking to figure out what your opponent did.
Thanks for the help guys really appreciate it.