Example of how to set the Square consent checkbox to be checked by default.

tw_ee_square_consent_checked_filter.php

<?php //Do not include the opening PHP tag if you already have one

function tw_ee_square_consent_checked_filter( $options, $form ) {
    if( $form instanceof EE_Billing_Attendee_Info_Form 
        && isset( $options[ 'name' ] ) 
        && $options[ 'name' ] === 'SquareOnsite_BillingForm' 
    ) {
        if(isset($options[ 'subsections' ]['consent_box'])) {
            $options[ 'subsections' ]['consent_box'] = new EE_Form_Section_Proper([
                'layout_strategy' => new EE_Div_Per_Section_Layout(),
                'subsections' => [
                    'consent' => new EE_Checkbox_Multi_Input(
                        [
                            'consent' => esc_html__(
                                'Allow Square to store my contact information.',
                                'event_espresso'
                            ),
                        ],
                        [
                            'required' => false,
                            'required_validation_error_message' => esc_html__(
                                'This will allow Square to store your billing information as a Client entity.',
                                'event_espresso'
                            ),
                            'html_label_text' => '',
                            'default' => 'consent'
                        ]
                    ),
                ]
            ]);
        }
    }
    return $options;
}
add_filter( 
    'FHEE__EE_Form_Section_Proper___construct__options_array', 
    'tw_ee_square_consent_checked_filter', 
    10, 
    2
);