Done! It was indeed pretty easy. But maybe for a non techie it may still be a bit hard to setup. For example, the model name was automatically set in the settings, but wasn’t the correct one. Luckily I recognized the model name in a curl example for Claude on the API page and then it worked 
Estimated costs are maybe 30 euro cents per month for spam control (I don’t have a huge forum). So that’s manageable! I’ve set a limit of 5 euros in the API console, just in case.