First, you insert your minimum acceptable salary. This is the value below which you think it would not be worth to take the job.
The previous step will return you a link that you can send to the recruiter. Don't forget to tell the recruiter the details about the number you inserted in the form!
With the link, the recruiter can now check if your salary expectation is compatible with the company budget.
The recruiter can then tell you if you're compatible or not.
Note that during the process you never reveal you salary expectation to the recruiter nor does the recruiter reveal to you the company budget!