In Affinity Designer one must often jump from vector domain to pixels. This is not an exception.
The basic idea is already told in comments and other answers. It's to make heavily gaussian blurred copy of your image and crop it. Unfortunately in vector domain the cropping also reduces the area where the blur is calculated. You wouldn't get full blur at the edges of the wanted area. You must
- make a copy of your image, all layers in the copy merged to the top layer
- add layer effect Gaussian Blur
- add a layer effect "color overlay". With low opacity it gives to your acrylic plate some plausibility. In the following example the color overlay is white and its opacity is 13%
- rasterize the layer, do not copy the layer effects (=apply the effects destructively, not any more editable)
- crop the layer to wanted size or make a pixel selection of the wanted area and copy&paste it as a new layer
EDIT: this is based on quessed original. it's much too dark. I have used the right original (by Microsoft) in the addendum. To get the same result the highlights in the right original should be flattened by the curves tool in a photo editor.
ADDENDUM: The questioner wanted a video. Unfortunately my screen recording system makes too large files to upload here. It also has a limited color palette, so the result is inferior for teaching image editing. Hopefully the workflow is understandable. Here's the link
Workflow demo video
As separate screenshot the result is the following: