Is it permissible for a Muslim to intend the Duha prayer as charity on behalf of his joints, and will he be rewarded for it if he does not intend anything, and likewise for the Sunnah rawatib prayers before and after the obligatory prayers?

Whoever prays the two rak'ahs of Duha with the intention of Duha has attained the reward of giving charity on behalf of their joints; due to the saying of the Prophet, peace and blessings be upon him: "And two rak'ahs that he prays in Duha will suffice for that." It is not a condition to intend charity for each joint. Similarly, the intention for the regular Sunnah is fulfilled by intending to perform it. So, whoever says "Allahu Akbar" intending to perform the regular Sunnah has fulfilled the intention. And if one prays without intending it, they will be rewarded for their prayer, and it will be an absolute supererogatory prayer.