Initially I thought doing this via the Proxies payload in the macOS section of the Profile Editor was the way to go (specifying a link to get the PAC file). Further research shows that authentication via PAC is really not supported for many proxy servers and it is the auth part that is what you are looking for (a way to automatically insert the username). Definitely, inserting the password wouldn't be a good idea, as its a security concern. That being said, it does seem like some proxy solutions do support this kind of thing (
I guess I still think that if your proxy server supports authentication via PAC, configuring the Proxies payload and pointing to your PAC would be the first avenue to go down.
Alternatively, take a peek at the Network payload... this may work.