The Affinity Photo box blur filter can be found in the blur category under the filter menu. It should be noted that the effect totally modifies the image in a destructive manner. You can also use the live effect which is non destructive so the underlying layer is untouched
Load your image and then go to the blur category and select the tool, there is only one option and that is the radius. You can use the slider or you can interactively modify the radius by dragging across the document. If you set it to a very low value such as 0.5 px and you will see a very subtle effect. Set it higher and you will see a linear effect (vertically and horizontally) appear. If you set it even higher then the image will become more like streaks across the image but you should still be able to make the image, put it to the slider max of 100 px you will be able to see the linear effect even clearer. Set it beyond that and the image will just become hard to see, creating a very strong mist. At the max of 1000px, it becomes basically the average tool.
You can apply the effect say using a radius of 50px and then you can fade it, you can fade the effect via the layer menu fade command and set a blending mode and opacity for the effect. Set the blending to color burn, difference, darken etc and many interesting images can be created. You can also fade the fade.
You can also create a duplicated layer via the layer menu duplicate command and apply the effect to that. The layer can also be rotated and re-sized and also can be blurred.
It can also be applied differently across different color channels, go to the channels panel found in the studio section of the view menu and select the red channel and then apply the effect with one set of settings and then select the green channel and apply different settings and so on
It can also be applied to a selection, you can use the select menu as well as any of the selection tools such as rectangular or freehand. With your selection created, run the it again and it will be applied only to that selection. You can then move or re-define the selection and then re-run it perhaps with a different setting.
It can also be applied to selected shape created via the shape tools as well as to text (artistic or frame) but as soon as you do this, the layer will become a pixel layer. If you set the value really low and it will result in a coarse pixel layer. Set the radius higher and the shape will become more ghostly and extended beyond its original dimension (you will clearly be able to see the linear). Set it to 300 or 400 etc and the shape will virtually vanish but leave a ghostly layer on top of the entire image. As with the fade, you can also use blending modes such as linear light (saturation, erase, divide etc) to create some interesting designs.
It can also be applied to a pattern layer as well as a pattern layer rasterized. Select in the layers panel and then apply the effect. I would suggest setting the mirror option (top left) to be on otherwise you will see breaks in your design.
It is also available as a live effect which can be connected to a layer and is non destructive (it can be edited at any point) or applied above the layer and works with all the layers. You can also bring in blending as well as opacity to the live layer. You can edit it by double clicking the entry in the layers panel. You can also duplicate the effect and change the blending modes for the application of the layer.
Select the flood select tool and use the settings as required say to select part of a face. With that selected, copy and paste that selection and remember to go to the select menu and remove the marching ants. Go to the effect and apply the radius using the interactive setting (the slider only goes up to 100). You can also manipulate the layers panel with the dialog open so then set the blending mode for the current layer to linear light to create a lovely glow effect.