I usually hook it on before putting the shoes on.
BUT...
With a bit of slack in the line you should be able to pull the spring back enough to slide the cable into the slot.
If you can not get enough slack I
I would be looking at the cable to be sure it can move freely in the sheathing.
Sounds like you loosened the adjuster nut, I. The main cable. (Is the e brake handle in or out) maybe loosen more. At some point you should be able to pull out cable and have a huge gap with no effort.
New video by Dana Nance
View attachment 1716247339View attachment 1716247340View attachment 1716247341
A tool like this might help make it easier.
View attachment 1716247343