how can I pop an input box for a specific choice?

[red 888]

In jenkins I'd like to do this:

parameters {
    choice(
            name: 'blah',
            choices: 'one\ntwo\ncustom',
            description: 'if you choose custom enter a custom number'
    )
}

So they have three options on the drop down, but it would be nice if when they select the "custom" choice jenkins pops an input box to type in raw user input.

Is this possible? I don't want to use user input during the pipeline run because that means they need to choose custom then wait for jenkins to get to the stage where it asks them for input.