تم تجميع بعض اسئله الاعضاء تحت عنوان ارجو الرد على هذا السؤال بسرعه

[mr_moh55555]

Yes, it is perfectly OK to use that here without a preposition. One of the meanings of that as a pronoun is "at which; in which; on which; with which; to which". That works within the relative cause not only as a substitute for the noun, but also "to express a relation of conformity, agreement, or identity, especially in reference to time." (Merriam-Webster Unabridged Dictionary)