Is it permissible to add advertising space on a website for a foreign client or to run advertisements for them on their website?

If the content and activity of the website are permissible, then there is no objection to designing it and adding advertising spaces, as long as the displayed advertisements are not inherently forbidden, and they are not the primary purpose of the website. The responsibility for the advertisements lies with the one who operates them and earns from them, so he must ensure that his earnings from them are lawful. If the content of the website is permissible, then it is permissible to design it and activate the advertising service, unless it is highly probable that the website will display forbidden advertisements. In that case, one must refrain from activating the advertising service.