I recently asked a question on 'form submission with a link' and received four answers. I cannot see on the site how to reply and say thanks to these people
If you look to the left of my answer, you will see what looks like a "tick". If you press it, it will mark my answer as the answer to your question. This is the best way to say thank-you to people who have given you a good answer, such as this answer. Only you can mark a response as the "correct" answer.
Also, on the left above the tick, there is a number with an up-arrow and a down-arrow. Click on the up-arrow when you see a good answer. You can do this even if you didn't ask the question. This is like giving the answer a vote - "I think this is a good answer to the question".
Hope this helps.
You say "thanks" in a few ways.
Most importantly, when an answer comes up that is most helpful to you, you click the checkbox by that answer to make it the "accepted answer." This give you and the answerer reputation points, which is a great way to say "thanks." You can only do this for one answer per question, so make sure it's the most helpful one.
If you come across an answer you agree with, or was helpful to you, you can click the up arrow next to the answer. This again gives the answerer reputation points, and also helps the community find answers that are helpful and correct. You can do this on any answer in any question, but it's especially nice to upvote people who give you good answers to your question.
If you want to thank the answerer personally, you can leave them a comment by clicking the "add comment" link under their answer. This is also how you ask clarification questions for their specific response.
Finally, if several people have given helpful answers, you can edit your question with an update that thanks the answerers who have contributed to your question. You can also update with whether you solved your issue, which is gratifying for answerers to see.
Here's your current SO account: https://stackoverflow.com/users/533513/johncron
I don't see any recently asked questions other than this one. This indicates that you've another account with which you've asked the question in question. That your current account is marked as "Unregistered User" is enough evidence.
It means that the account tied to only a specific PC/webbrowser by a long-living cookie. If you want to regain the original account, then you need to come back to Stackoverflow using exactly the same PC/webbrowser as you've asked the question with and then register the account by OpenID. This way you'll be able to login using the same account from every PC/webbrowser in the world. You will then also be able to post comments on answers and mark the best one accepted.