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:

opt1 opt2 opt3

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.