Use a prefix containing the delimiter

[micarmst]

I'd like four blocks of three numbers separated by a hyphen. The first two blocks of numbers are fixed (see the prefix) and the user only provides the last two blocks of digits:

var cleave = new Cleave('.input-custom', {
    prefix: '785-223',
    blocks: [3, 3, 3, 3],
    delimiter: '-',
    numericOnly: true
});

https://jsfiddle.net/fjmh9Lwt/2/

However this does not display correctly and I'm also unable to type in to the textbox. I thought maybe the prefix cannot contain the delimiter so tried removing it and relying on the blocks to add it. Although this way displays correctly I still can't type in to it:

var cleave = new Cleave('.input-custom', {
    prefix: '785223',
    blocks: [3, 3, 3, 3],
    delimiter: '-',
    numericOnly: true
});

https://jsfiddle.net/fjmh9Lwt/3/

Any ideas on how I can achieve this?

[Helox]

Same, using prefix like 'W' works but 'W-' does not, input blocks and typing does not change the value.