Is imagining meeting a specific man and marrying him and living a happy married life with him considered forbidden, and is specifying a particular person to imagine as a husband considered forbidden?

Fleeting thoughts arising from self-talk or imagination do not incur sin. However, dwelling on them and indulging in them is not appropriate, as it exposes the soul to provocations and satanic whispers and incites the commission of forbidden acts. The Prophet, peace and blessings be upon him, forbade a woman from describing another woman to her husband. It is more befitting for a Muslim woman to occupy her time with what benefits her in her religion and her worldly life.