Startup disk doesn't show my windows partition Macbook Pro late 2011 running High Sierra (10.13.6). I have swapped my HDD for an SSD and placed my HDD instead of my optical drive. 

I would like to install a Windows 7 partition on my SSD using bootcamp assistant. For that I have booted a windows 7.ISO on a USB (External physical volume), and successfully created a windows partition with bootcamp assist. Now during the restart I had a black screen saying "...no bootable disk found..." even though my usb was plugged in. I forced manually a restart pressing alt, i can only select my os partition. 

Now on my start up disk in system preferences I only see Macintosh SSD. How can I get my macbook to force restart on my Windows Partition and read my usb stick?

PS: I downloaded my bootcamp drivers and have them on a separate USB + on my OS desktop
A previous answer, which will get Windows 7 installed on your Mac, can be found at Installing Windows 7 or 10 Pro 64 bit without DVD or Flash Drive.

If you have the Windows 7 product key, then you should make sure you have the latest Windows 7 iso download. You can get this from Download Windows 7 Disc Images (ISO Files).

I will point out that the Windows 7 product key can also be used to install Windows 10 for free. The latest Windows 10 can be found at Download Windows 10 Disc Image (ISO File). Instruction for installing Windows 10 can be found at How to install Windows 10 into a 2011 iMac without using the Boot Camp Assistant, an optical (DVD) drive or third party tools?.

Addendum

You could also probably install Windows 7 by first booting from a Windows 10 USB installer. This would then require using the CLI to install Windows 7 to an internal drive. The instructions would be similar to this answer. If interested, I could include updated instructions.