Is it possible to apply_filter on a wp_ajax_ action?

The question is: Is it possible to hook with apply_filters or
add_filters on their function, manipulate it adding HTML to that page?

apply_filters and add_filter() do two separate things. apply_filters allows a value to be filtered by add_filter(). This means that if the original code is 3rd-party, you can only use add_filter() if that code already has apply_filters() applied to it. If you’re extending another plugin, you will be using add_filter(), not apply_filters().

So you need to check if the original checkAddons function includes any filters for you to use, by seeing if it uses apply_filters() anywhere. If it does, then you can filter the value passed to it with add_filter(). If the filter is properly supported, then ideally the original developer has documented it somewhere.

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