Custom card

Hi everyone, 

i want create a custom card to show one specific value when the data to show is (Blank).

My idea is create an input of text in format section, to replace in visual when the data is null.

With i have at this moment appear the name of option in format but no the textfield and i dont know what i must write to it work.

I am a bit new in this creation of custom visuals,so please apologize me if you see basic errors.

Thanks in advance!

At this moment i have this:

capabilities:

{
    "dataRoles": [
        {
            "displayName": "Field",
            "name": "field",
            "kind": "Measure"
        }
    ],
    "objects": {
        "blankText": {
            "displayName": "Blank Text",
            "properties": {
                "description": {
                            "type": {"text": true},
                            "description": "New value to change (Blank) value "
                        },
.....
"dataViewMappings": [
        {
            "conditions": [
                {
                    "field": {
                        "max": 1
                    }
                }
            ],
            "single": {
                "role": "field"
            }
        }
    ]
}

visual: