Yes, if it's unavoidable, you can go into a rental agreement after you move into the property. However, in such a case, it is utterly important to ensure that the landlord is trustworthy. If you are paying a lump sum amount, make sure you do it through online transfer or via cheque. However, it is highly recommended to get the rental agreement done as soon as possible. It can be quite risky to continue the entire term without any legal agreement. And, in case, the owner of the property or the landlord is refraining from making the agreement even after 30 days of your stay, ask him to get it done. And, even then if he does not bother to do the same, stop paying the rent immediately and move out of the premises.