Sending a response after Form completed

Hi Learning_fun 

I'm not quite following what you mean by giving the responder the option to tick Send me an email receipt of my responses. I am not aware of that being a function of Forms unless you mean that if they tick a box in the survey, you would then manually, or using Power Automate, email their results to them?

To your question, if the email address has been recorded in their submission, you could manually look at the Email addresses of the responders up to the time you had switched on the Send email receipt to respondents option in your form, and send them a BCC email thanking them for their input. 

By switching on Send email receipt to respondents, all future responders will receive the acknowledgement email.

If you are looking to send them the answers that they have submitted, you'll need to use Power Automate to achieve this for future responses, And while you could rig something in Power Automate to redo the previous responses too, it will need a bit of work.

Hope that all makes sense?

Cheers

Damien