First you will have to apply for the IPO using Dhan (app or website). After the application, there is a request to block the funds that are sent by the exchange via UPI mandate, on the UPI application to which your associated bank account are mapped.
You will receive a request to block funds equivalent to the amount applied, on authorisation the said amount is blocked. In case of successful allotment, there will be a subsequent debit of funds. If there is no allotment, the blocked funds will be released.
Note : Fund Transfer to Dhan Account is not required in case of IPO as it is UPI linked Transactions.