EA_Tom over at Answers HQ has posted that Origin in-game will no longer function in Mojave. This is due to the increased functionality in the macOS System Integrity Protection. Tom’s post is here and you should definitely go and read it – https://answers.ea.com/t5/The-Sims-Mac/Origin-In-Game-and-the-macOS-Mojave-Update/m-p/7053780#M14299
There are detailed instructions in the post on how to bypass System Integrity Protection in order to get Origin in-game to work. If you decide to do this you MUST re-enable it again immediately after closing Origin. This OS feature is an integral part of your Mac security. Disabling and re-enabling it requires restarting your Mac.
For the record, I won’t be doing this and will simply have Origin in-game disabled (as I do now). My suggestion would be not to do it either as it can seriously compromise the security of your Mac. Google disabling System Integrity Protection if you want more opinion.