There is no way to protect your code to work with freelancer, but you can put some efforts in reducing the risk. I understand the issue since I am a freelancer and sometimes potential clients are concern about their code to be stolen.
Here my advice:
1) Work only with freelancer that have a good reputation and that can prove they have the skills to help you. If they are more skilled then you, they don't need to steal your code since they can for sure make the game all by themselves and probably better. That's why you ask their help in the end, right?
2) If you need a specific function, just ask to make a standalone program. You can just copy the Event Sheet(s) provided into your game and adapt them.
3) If solution n.2 doesn't work for you, then create a new project, recreate the issue you need to fix there with placeholder art and audio, and ask to fix the issue in that test program. You will then copy and adapt it to your game.
4) If solution 2 and 3 are not suitable for you, then make a copy of your program and remove from it all the things that are not related to your issue, make sure the issue is still happening as in the original program and then send it to the freelancer.
For example, if you need a level selection layout, you can simply remove all the event sheets of the game, leave just the layouts with their name, but empty of all the art and objects and give the skeleton of your program to the freelancer that will create the event sheet and layout for level selection.
5) This might be slower and therefore more costly, but it's a solution sometime I offer to clients as well. Pay the freelancer per hour and go on video call, share the screen and show the issue and the part of code where you think the freelancer might find the problem. The freelancer will takes a bit of time to study the code and trying to figure out where the issue is, depending on the skill of the freelancer and how much your code is organised, long and complicated. Then together you will fix the issue step by step.
Those tips should give you a good amount of protection, if you are looking for a freelancer you can get in contact with me, I am a bit busy with reforming my house at the moment, but I could be able to help you depending on what you need.