You'll have to download iTunes from Apple first, then run PlayOnLinux.
Picture guide:
- In the PlayOnLinux window, select "Install a Program."

- Then, select "Install a non-listed program" in the bottom left.

- Click through the agreements and such and then select the "Install a program in a new virtual drive" option and click Next.

- Name the drive iTunes and continue.
- Next, check the options, "Use another version of Wine" and "Configure Wine" and continue.

- Choose System for the Wine version and click Next.
- Choose the 64-bit system option on the next screen and continue.
- Let Wine do its thing until a window pops up. In this window, change the Windows version to Windows 7.

- Click OK and then browse to the iTunes installation you downloaded previously, then click Next. iTunes should install.
There is no guarantee of this working correctly, as iTunes is not meant to run on Linux, but there are rumors of it starting up fine. Even if the installation is successful, there will be two errors when you try to open iTunes. According to the report on WineHQ, these errors are mostly meaningless and can be ignored. This report also states that iTunes takes a while to start, but for me, it never does. Good luck.
The report: https://appdb.winehq.org/objectManager.php?sClass=version&iId=31322&iTestingId=89752