It is forbidden to ask a married woman for her hand in marriage by the consensus of all Muslim scholars. The consensus has been transmitted by Ibn Taymiyyah and al-Shirbeenee.

It is not allowed to propose to a woman during her waiting period using explicit words [4] Asking for her hand in marriage in a way that leaves no room for doubt that it is a marriage proposal like saying, will you marry me, or saying once the waiting period ends, I will marry you, and so on. by the consensus of all Muslim scholars. This consensus has been transmitted by Ibn Hazm, Ibn ‘Atiyyah, Ibn al-Qattaan, and Ibn Taymiyyah.
b) Implicit Marriage Proposals: 

This refers to words that can be understood as a marriage proposal or anything else.
i. Examples of implicit proposals: I am interested in you! Once it is over, I will make my move! I do not want to lose you, and so on. These statements and their likes are considered implicit words that could be a marriage proposal by the agreement of the four Madhabs.
ii. Ruing of proposing to a widow during her waiting period using implicit words: It is allowed by the agreement of all Muslim scholars, as reported by Ibn Hazm, Ibn ‘Abd al-Barr, al-Baghawee, Ibn al-Qattaan, Ibn Hajar, al-’Aaynee, and Ibn Najeem.
iii. Ruling on the widow responding to the marriage proposal during her waiting period using implicit words: It is permissible according to the majority of scholars, namely the Maliki, Shafi’i and Hanbali scholars.
iv. Ruing of proposing to a divorcee during her revocable divorce waiting period using implicit words: It is forbidden by the consensus of all Muslim scholars, as reported by al-Qurtubee, Ibn al-Humaam, al-Mardaawee, al-Haskafee, al-Zarqaanee, and al-’Adawee.
v. Ruing of proposing to a divorcee during her irrevocable divorce waiting period using implicit words: It is permissible according to the majority of scholars, namely the Maliki, Shafi’i and Hanbali scholars. Some scholars reported that this is a matter of agreement by all scholars.
c) Ruling of a man explicitly asking his ex-wife’s hand in marriage [during her waiting period] if separation is irrevocable, but it is not irrevocable due to issuing three divorces: [5] The Maliki scholars state that it is disliked for a person to marry a woman that he explicitly asked her hand in marriage during her waiting period. It is permissible according to the agreement of the four Madhabs. 
d) Ruling of marrying a woman whose waiting period has expired but proposed to her during her waiting period, explicitly or implicitly:

The marriage contract will be valid, in both cases, even if it was not allowed for him to propose to her during her waiting period, so long as he marries her after the end of her waiting period. This is the view of the Maliki, al-Shafi’i and Hanbali scholars, and it is the apparent view of the Hanafi Madhab. [6] Hanafi scholars do not mention that marrying a woman during her waiting period is prevented.