nf-custom-action-example.php

<?php

/**
 * @tag my_ninja_forms_processing
 * @callback my_ninja_forms_processing_callback
 */
add_action( 'my_ninja_forms_processing', 'my_ninja_forms_processing_callback' );

/**
 * @param $form_data array
 * @return void
 */
function my_ninja_forms_processing_callback( $form_data ){
    $form_id       = $form_data[ 'form_id' ];

    $form_fields   =  $form_data[ 'fields' ];
    foreach( $form_fields as $field ){
        $field_id    = $field[ 'id' ];
        $field_key   = $field[ 'key' ];
        $field_value = $field[ 'value' ];
        
        // Example Field Key comparison
        if( 'my_field' == $field[ 'key' ] ){
            // This is the field that you are looking for.
        }
    }

    $form_settings = $form_data[ 'settings' ];
    $form_title    = $form_data[ 'settings' ][ 'title' ];
}

I'm trying to write my own payment api, using ninja forms. Where would I save my custom action function?

https://github.com/wpninjas/ninja-forms/issues/3502

I'm trying to write my own payment api, using ninja forms. Where would I save my custom action function?

You should save the action in your themes' function.php file. Further detail is here

Dear link. What's up with all the bad quality documentation in this project? Lots of docs, but low quality.