Hello,

Unfortunately, I receive around 50-100 SPAM requests every day via my classifieds market, or rather someone clicks on my ad and then uses the contact request as spam, I think it would be very good if a captcha could be installed, is that possible?

Yes, there are free & paid plugins, even Osclass itself has built-in feature for this

Hello,

and how can I activate the integrated function?

Check documentation.
Settings > Spam