BakkesMod is a software that gives you access to a lot of plugins for Rocket League. As the Workshop Map Loader & Downloader is a plugin, you need to have BakkesMod to get it.
Follow this link https://bakkesplugins.com/ > BakkesMod > Download BakkesMod
Once you've downloaded it, install it.
Then open BakkesMod and Rocket League, check that BakkesMod says "Injected"

Once you know BakkesMod has been injected in the game, press F2 in RL to open its interface.
Go to the Plugin Tab, then PluginManager > open PluginManager > Enter the ID "223" and press Install by ID.
You know have WorkshopMapLoader appearing among your plugins

Select the WorkshopMapLoader plugin > Open Plugin Window > Search Workshop > type "Credit Agricole" > Search
You should see the map and click on Download Map
Once the map has been downloaded, go to the Map Loader tab > Refresh Maps > select the map > Solo
You can now play the map