Net banking has IPO application form. You can go there and apply by giving your DMAT account details which is linked to your bank.
Once you submit the application, the amount will be blocked till the IPO window. In case you are allotted with the shares then the amount will be debited from your account else it will be credited back.
HDFC AMC IPO has a grey market premium of 300, it means it can pen at 1400. Hence it's worth applying for.