Does a percentage of the lease contract become obligatory for the website after its completion, knowing that the advertiser was unaware of the condition until after reaching an agreement with the tenant?

Your lack of knowledge regarding the website's condition for a brokerage and referral fee does not exempt you from paying it. You are negligent for not reading the terms. Website terms are legally binding on their users. If the custom dictates that advertising websites are entitled to a fee for referral, then they are entitled to it even if it is not explicitly stated. It has been mentioned in the fatwas of the Permanent Committee that the referral fee is based on prevailing custom, and if knowledge and mutual consent occur between the contracting parties contrary to the custom, then there is no harm; otherwise, it is not permissible because the wealth of a Muslim person is not lawful except with his good will.