Changing the "Add Item" button text in Sanity.io using a custom inputComponent.

{
    name: "categories",
    title: "Categories",
    type: "array",
    of: [{ type: "category" }],
    options: { buttonTitle: "Add new category" },
},

"parts": [
    {
        "implements": "part:@sanity/form-builder/input/array/functions",
        "path": "./parts/customArrayButton.js"
    },
    ... // other parts
]

import React from "react";
import { isReferenceSchemaType } from "@sanity/types";
import { AddIcon } from "@sanity/icons";
import { Button, Grid, Menu, MenuButton, MenuItem } from "@sanity/ui";
import { useId } from "@reach/auto-id";

const customArrayButton = React.forwardRef((props, ref) => {
	const { type, readOnly, children, onCreateValue, onAppendItem, value } = props;
	const menuButtonId = useId();
	const insertItem = React.useCallback(
		(itemType) => {
			const item = onCreateValue(itemType);
			onAppendItem(item);
		},
		[onCreateValue, onAppendItem]
	);
	const handleAddBtnClick = React.useCallback(() => {
		insertItem(type.of[0]);
	}, [type, insertItem]);
	if (readOnly) {
		return null;
	}

	return (
		<Grid
			gap={1}
			style={{ gridTemplateColumns: "repeat(auto-fit, minmax(100px, 1fr))" }}
			ref={ref}
		>
			{type.of.length === 1 ? (
				<Button
					icon={AddIcon}
					mode="ghost"
					onClick={handleAddBtnClick}
					text={type.options?.buttonTitle || "Add item"}
				/>
			) : (
				<MenuButton
					button={
						<Button
							icon={AddIcon}
							mode="ghost"
							text={type.options?.buttonTitle || "Add item…"}
						/>
					}
					id={menuButtonId || ""}
					menu={
						<Menu>
							{type.of.map((memberDef, i) => {
								const referenceIcon =
									isReferenceSchemaType(memberDef) &&
									(<http://memberDef.to|memberDef.to> || []).length === 1 &&
									<http://memberDef.to[0].icon;|memberDef.to[0].icon;>
								const icon =
									memberDef.icon || memberDef.type?.icon || referenceIcon;
								return (
									<MenuItem
										key={i}
										text={memberDef.title || memberDef.type?.name}
										onClick={() => insertItem(memberDef)}
										icon={icon}
									/>
								);
							})}
						</Menu>
					}
				/>
			)}
			{children}
		</Grid>
	);
});

export default customArrayButton;