I need to make a Start-up disk on a USB Drive for my iMac that only has a White Start-up screen

If your Mac starts up to a blank screen

To perform this action will require booting from a Bootable Installer. 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of El Capitan would have to be done on a computer that Qualifies to run El Capitan .

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

You are welcome and thank you too.

Do come back to the Apple Support Communities ( ASC )  in the future, if / or when additional questions may arise