menu Select>Save Selection
- Record action with Relative turned on and type some values.
- After recording action click on modal on the left side of action step: Canvas Size. This will force Photoshop to open Canvas Size dialog when playing action while Relative will be turned on so you need to change values and click OK.
Here is sample action I have recorded Relative.atn - Google Drive
I have recorded the action with pixels as a measurement unit. You can record the action in some other measurement unit if you are working with inches, for example.
And by the way, you can assign a keyboard shortcut to play action.
Photoshop Grays out items in menus when the item can not be used for some reason. In my first screen capture every item in menu Select is grayed out because Canvas size in being used. On your Mac I see ALL seems to still be possible. My second screen capture you will see that some Item in the Select menu are useable to create selection but Save Selection is grayed out because the is not active selection. In my third screen capture Save Selection is available and I can use my shortcut key Alt+Shift+Ctrl+F10 this also shows I'm using Windows which has Ctrl not CMD and Alt not Option.
Thanks . That did help