Update Multi-Select from Single Select

I have a multi-select field with 10 options
I have a single select with the same 10 options
I want to run a script to take the value in single select and update the multi-select with it.

For example, if the values in a record before running the script are:

  • Multi-select: Apple, Bat
  • Single-Select: Car

After the script executes, I would like to see this:

  • Multi-select: Apple, Bat, Car
  • Single-Select:

Thanks in advance!