Use the following code to set a custom invoice number based on a field value. Replace 25 with your form ID and replace 123 with the ID of the field that should set the invoice number.

add_filter( 'frm_authnet_invoice_num', 'set_custom_auth_invoice_num', 10, 2 );
function set_custom_auth_invoice_num( $invoice_number, $args ) {
    $field_id = 520;
    $field = FrmField::getOne( $field_id );
    $entry_meta = FrmEntryMeta::get_entry_metas_for_field( $field_id );
    
    if ( $field->form_id == '45' ) {
        $invoice_number = $entry_meta[0];
    }   

    return $invoice_number;
}