If you want to change the default: Folder Formats: Quick Guide
On the other hand, if you don't want to change the default and only want your custom format in special situations, turning on the Format Lock (padlock icon in the status bar) will lock whatever your current format is for that file display, and that carries over when you open new tabs in the same file display.
The format lock state is saved with layouts, so you could save a layout with it on and your custom format set. Or you could make a button which both applies the custom format and turns on the lock. (Or various other things, if none of those work.)
If you need help setting any of those ideas up, let us know.