Revolution Slider - Post Based Slider with ACF Custom Field Background Image

Question

Anyone managed to get RevSlider Post Based to show a custom field image as background instead of the default featured image?

I have set up a few custom fields on a CPT and everything shows up ok. But, I need the Background image to be an image set in a custom field instead of the featured image. I had also tried it by selecting External Image as a source and adding {{meta:my_custom_img_url}} to the URL but it does not work :).

Any help is appreciated!

Thanks