Hello, is it possible to add spaces at the end of a prefix via a command?

What I want: [test] Bob: blah
What I get: [test]Bob: blah

This is the command I use: /lp user Bob meta addprefix 50 '[test] '

Note: The 's aren't there, they're simply to show the space at the end.

You should use double quotes, not single ones. Try /lp user Bob meta addprefix 50 "[test] "

As voodootje0 said wrap it in ": /lp user Notch meta addprefix 1337 "[We love you] "

Alternatively just add the space in your chat format.