.ext-sign-in-box {
    position: relative;
    padding: 20px;
    background: white url("Your-Function-URL") center no-repeat;
    min-height: 300px; /* Set a minimum height to ensure the error message is visible */
}

/* Optional adjustment to ensure the image scales correctly */
.ext-sign-in-box {
    background-size: contain;
    background-repeat: no-repeat;
}