The iPad version does not have the option to load a selection file like the desktop version but you can save selections as spare channels and load as a pixel selection as required.
Hi Chaz,
Welcome to Affinity Forums :)
As Callum said this feature is currently only available in Affinity Photo (convert paths to selections and masks).
Here's a tip to work around the problem in Designer: draw your shape with the Pen Tool as usual, fill it with any colour and no stroke, then to "convert" it to a selection press and hold ⌘ (cmd) and click over the object's thumbnail in the Layers panel. Affinity will create a selection from the layer contents.
