Takes ownership of value and sets it for name to form.
The value will be freed with g_free, when no longer needed. The form should be the
one used in prepare_authentication_uri_query,
prepare_get_token_form or
prepare_refresh_token_form.
If the value is null, then the property named name is removed from the form instead.
| form |
a GenericSet |
| name |
a property name |
| value |
a property value |