Include the key-password argument when issuing the command, as follows:
.
fglam.exe --silent --configure --downstream port=443,type=https,key-password=<password>,size=Huge --auth-token encrypted:....
.
Following is an example with the password set to "testpasswd" and an auth-token supplied
fglam.exe --silent --configure --downstream port=443,type=https,key-password=testpasswd,size=Huge --auth-token encrypted:e33e6e3537a0792ff66fab4b36ea19bab317ad76e03b044308935df8e7f0efab902de6275b2da09880eeebabd643a55d31986b043c47db1c57d0612b5b25a66e099b1147adb1cf0ca5593ba55607e326b58afbed5dbf31bba6c3d62c31758ebcf0d8941b6a317b41b93f72c4d18c2919