Where can I find the Field ID for a header and that of text.

The header and text elements do not have a field ID listed on their properties as they do not receive any input. 

If you need to get the fields IDs then you will need to inspect them through the web developers tool, to do it, simply load the direct link of your form and right click over the element you want to get the ID, select the inspect option in order to get the HTML elements, you will be able to get the elements properties, including their ID. 

I hope this helps.