Illustrator Scripting Help.

Hello @Kelly33449404c58c, I just saw your request.

I've created a script that will do as requested. I wasn't sure of the intended purpose, so I've modified it to allow you to change the amount of padding on the new artboard, defaulting to 7 points. It can even use negative values to make the new artboard smaller.

Save the attached file to your computer and change the extension from .txt to .jsx.
In Illustrator, run it using File > Scripts > Other Scripts ..., select the downloaded script and click Open.

Cheers,
Richard

#target illustrator

// This script adds a new artboard to the active document and centres it under the original artboard with a specified border width.
// It is in response to a question on Adobe Community: https://community.adobe.com/t5/coding-corner-discussions/illustrator-scripting-help/td-p/14277937
// Written by Richard Turner-Jones (Community Expert) 

function addCenteredArtboard() {
    var doc = app.activeDocument; // Get the active document

    if (!app.activeDocument) {
        alert("No active document found.");
        return;
    }

    if (doc.artboards.length === 0) {
        alert("The document must have at least one artboard.");
        return;
    }

    // Prompt the user for the border width, defaulting to 7 points
    var borderWidthInput = prompt("Enter the border width.\nPositive numbers will make the new artboard larger.\nNegative numbers will make the new artboard smaller.", "7");
    var borderWidth = parseInt(borderWidthInput, 10);

    // Validate the user input
    if (isNaN(borderWidth)) {
        alert("Invalid input. Please enter a number.");
        return;
    }

    var originalArtboard = doc.artboards[doc.artboards.getActiveArtboardIndex()]; // Get the current active artboard
    var originalRect = originalArtboard.artboardRect; // Get the coordinates of the original artboard

    // Calculate the width and height of the original artboard
    var originalWidth = Math.abs(originalRect[2] - originalRect[0]);
    var originalHeight = Math.abs(originalRect[1] - originalRect[3]);

    // Define the width and height of the new artboard, including the border width
    var newWidth = originalWidth + borderWidth * 2;
    var newHeight = originalHeight + borderWidth * 2;

    // Calculate the position of the new artboard to be centred under the original
    var newX = originalRect[0] - borderWidth;
    var newY = originalRect[1] + borderWidth;
    var newRect = [newX, newY, newX + newWidth, newY - newHeight]; // New coordinates for the new artboard

    // Add the new artboard
    var newArtboard = doc.artboards.add(newRect);

    // Optional: Set the new artboard as the active one
    doc.artboards.setActiveArtboardIndex(doc.artboards.length - 1);

    alert("New artboard added and centred under the original with a border width of " + borderWidth + " points.");
}

// Run the function
addCenteredArtboard();