Is it possible to put Windows 10 onto an external SSD and boot it through VirtualBox/VMware? I am getting a 13-inch Macbook Pro (possibly the 2019 model) on my birthday. I am wondering if I can put Windows 10 onto an External SSD and boot it with virtual machine software on my Mac (ex. VMware, VirtualBox)
In a word, yes. you can place the VM storage (EG the HD file) anywhere you want it.

Note that both programs have default locations to place the VM files so you will likely have to specify advanced setup and specify the location manually.