A short guide with images on how to enable server resource packs for a server where you denied it before!
In your multiplayer serverlist select the entry of the server for which you want to enable server resource packs.
Click on the "Edit" button on the bottom left to open the server entry's options menu.
There you can set "Server Resource Packs" to "Enabled".
Click the "Done" button and server resource packs are now enabled for that server!