NI38 Form - how to pay one-time Class 3 contribution from abroad?

Hi, I left the UK decades ago and want to make a one time Class 3 contribution to cover my shortfalls to fulfill the minimum requirements for a UK pension. I want to decide later whether to pay also ongoing Class 3 contributions. How do I have to fill the NI38 form for such one-time contribution? Do I need to fill Section 4 "Self-employed/employment abroad". It seems that has not relevance in case of Class 3 contributions? Do I need to fill section 6 "Application to pay voluntary contributions" or only tick Section 7 "Shortfall of National Insurance contributions"? I fear I get charged automatically in the following years if I fill section 6. Thank you for clarifying.

Hi Gerrit,
If you complete and return the CF83 application form you will be able to receive details of any shortfalls of voluntary National Insurance contributions that you may pay.  
If you do not wish to set up either a direct debit or annual request for payment going forward then leave questions 23 & 24 blank.
If you’d rather just send us a letter to ask to pay voluntary contributions rather than complete the CF83 application form then this would be fine.
Before asking about paying voluntary contributions you should check with the Future Pension Centre to make sure that it is worthwhile for you to pay them.  
Their contact details can be found via:  Check your State Pension forecast
Thank you.