Question

what preposition is used before the name of institute ?

Answer 1

The is used.
For example
The unison institute

Answer 2

Prepositions are used in a sentence before any word, be it a name, as per requirement. There is no such hard and fast rule. I think you have mistaken the question by asking "preposition" instead of "article". As per articles are concerned, as the name of an institute is a proper noun, it does not take any article. However, if the name itself has an article in it, then that article is compulsary to be used.   Hope this helps!!!..:)