When should I fill the cash field form related to fee payments for my visa application in UK/Ireland?

There is no specific "cash field form" required for UK or Ireland student visa applications.

For the UK, you pay the visa application fee and Immigration Health Surcharge (IHS) online when you submit your visa application. You must also show proof of funds (usually a bank statement showing the required amount held for 28 days) as part of your application.

For Ireland, you pay the visa fee when applying and a separate GNIB registration fee after arrival.

If you meant a different form or requirement, please share more details with your LeapScholar counsellor.