Custom Editable Javascript init method not called


I am currently creating a custom datetime editable which runs on a bootstrap 4 plugin. Everything works fine, except that I cannot get the data from the PHP-Backend (the date) into the bootstrap 4 plugin to be displayed in the input field.

I thought I had to do this in the initialize method of the editables javascript file, but this method is never called. Does anybody have an explanation for this or a documentation about creating editables besides the one online which says almost nothing on this topic?