My mirc when I join an existing room doesn't show the previous history automatically. I type /showlog #channel name

Is there a way I can use the ini for something like
On join
Command

Or perform an on connect for the server?

I do have a bunch of rooms I open up everyday that I could make a list for but I'd rather there be a way even if I enter a room I haven't been in before it automatically runs show log.

Try this

Tools > Options > click on [+IRC] to expand the list and look for "Logging".

"Automatic log and reload:" on the right side, is what you're after?