Is it permissible to apply an herbal sexual stimulant medication to one's wife without her knowledge, and is there a supplication (dua) that would make her responsive to a request for intercourse?

A wife must respond to her husband for intimacy, and it is not permissible for her to refuse without a valid Shar'i excuse; otherwise, she would be sinful. The husband can supplicate to Allah to inspire his wife to be rightly guided and to enable her to obey him. Among the suitable supplications are: "O Allah, I hope for Your mercy, so do not abandon me to myself even for the blink of an eye, and correct all my affairs for me. There is no god but You," and "There is no god but Allah, the Almighty, the Forbearing. There is no god but Allah, Lord of the heavens and the earth, and Lord of the Magnificent Throne," and "O Allah, there is no ease except what You make easy, and You make the difficult easy if You wish." If the wife refuses, thinking that she is not obliged after fifty or out of shyness, it must be clarified that this is not a valid Shar'i excuse. As for administering medicine to her, there is no harm in it if no harm is feared for her and she has no excuse for refusing intercourse, by analogy with the permissibility for a man to take what strengthens his sexual ability.