Scraper detects all custom fields automatically. You need to create empty post with filling all custom fields. Scraper will capture all custom fields from this empty post.
Plugin is unable to detect all custom fields because of WordPress’s limitations. But creating empty post, plugin can detect all variable names.
You can see your custom fields on data fields list with creating a new data field. Custom fields can contain multiple values. In that case you should also enable JSON formatting in transform settings > database.
Some plugins stores information in separated databases. For example GeoDirectory plugin shows values as a custom field but they are actually in a separated database. In those cases Scraper won’t shows these fields. Please make sure the plugin you use on your website stores information on custom fields.
You could also define custom field name on Transform > Database section. There is a field called “Database Name”. With changing it, you could update your custom field key.
Plugin doesn’t support some fields on Advanced Custom Fields plugin. (For example repeater data type)