Is it permissible to announce fasting on forums and offer supplication for others in return for that?

The texts of Islamic law affirm the desirability of concealing good deeds, as it is closer to sincerity (ikhlas (definition)) and further from self-admiration (ujb) and showing off (riya'). This is unless there is a predominant benefit in making them manifest, such as when the person serves as a good example.

Al-Hasan (definition) al-Basri praised those who secretly memorized the Qur'an, gained understanding of the religion (fiqh), and performed good deeds, citing the Almighty's saying: "Call upon your Lord humbly and privately," and the Almighty's saying about Zachariah: "When he called upon his Lord a private call."

As for showing one's deeds, it is commendable if it serves as a good example for others or provides a general benefit, such as writing and disseminating knowledge.

Regarding showing one's fasting for the purpose of supplication, there is no benefit that necessitates disclosing it, because supplication can be made secretly, and sincerity can be maintained. Furthermore, an angel may supplicate for him with the like of what he supplicated for his brethren. Therefore, concealing one's fasting is more preferable and beneficial for the heart.