See HTML form tutorial and HTML spec.
Text inputs: A text:
A password:
Radio set 1: val1
val2
val3
Radio set 2: val1
val2
val3
Checkboxes: val1
val2
val3
HTML5 inputs: A number:
A date:
A color:
A range:
A month:
A week:
A time:
A datetime:
A datetime-local:
An email:
A search:
A tel:
An url:
Files: A file:
Multiple files:
Dropdown:
Text area: Text area default text.
Submit:
Submit to Lua script: This will only work if server side Lua scripting is activated and /handle_form.lua can be found on the server.
Submit to callback: This will work in the embedded_c example. It will call mg_handle_form_data to parse the request.