What's that? Do you mean this?:
This plugin will allow you to automate various tasks using "Macros". Macros consist of a list of conditions under which a list of actions will be performed. Examples and guides can be found in the wiki. Feel free to contribute! If you run...
obsproject.com
If so, then the first way that comes to mind is to use the Date condition in two separate macros. One to start recording a little bit early, and the other to stop recording a little bit late. (better to capture too much than not enough) Leave everything set up and OBS running, and it should be there when you get back to it.
Of course, if you need to log in to see it, and you can't log in until a short time before it starts, like I've seen webinars do, you'll need to solve that separately.