Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear sister, thank you for trusting us and for your question that shows your interest to know more about Islam.
Though not obliged to, a Muslim husband should tell his wife about his new marriage, as this is a serious decision that will greatly affect the whole family. A husband should wisely and kindly handle such a sensitive issue.
Also, though it is permissible for him to marry a second wife in some circumstances, he is still required to maintain a good relationship with the first wife.
We should also draw your attention to the fact that it is not allowed for a woman to ask her husband to divorce his other wife. It was narrated that the Prophet (peace and blessings be upon him) said, “No woman should ask for divorcing her sister (fellow wife) to empty her bowl.” (At-Tirmidhi)
In this hadith, the Prophet (peace and blessings be upon him) considered the fellow wife as a “sister” to remind each wife of the ties of kinship, ordering her not to be selfish taking exclusive possession of the husband and leaving her fellow’s bowl empty. This is a Prophetic rhetorical expression meaning a wife takes all the benefits and leaves nothing to her fellow wife.
Responding to your question, Dr. `Abdul-Fattah Edris, Professor of Usul Al-Fiqh, at Al-Azhar University, stated,
The husband is not obliged to tell his wife before divorcing her and it is not necessarily for the wife to be present during the divorce. The Prophet (peace and blessings be upon him) deserted his wives for some time (about a month) without telling them.
Moreover, divorce can be applied through any considerable secure means; a written message, a phone call, an email, etc., as long as it is verified it is from the husband in person.
Allah Almighty knows best.