Is it permissible for a wife who has lost her husband and does not know whether he is alive or dead to supplicate with the prayer "My Lord, do not leave me alone [without an heir], while You are the best of inheritors" with the intention of his return?

We ask Allah to grant you well-being and reunite you with your missing husband. There is no objection to the aforementioned supplication in principle, as intercession through the Qur'an and supplication with all its invocations are permissible. However, the intention behind this supplication (in the case of Zakariyya) was to pray for a child. So, if you supplicate to Allah for your husband's preservation and well-being, that would be better.