You may need to add default values in contact fields when contacts submit forms.
To do so, create or edit a form (components > forms).
In the fields tab, add a new field (any type you want).
In the general tab of the form field you just added, fill the "default value" field with a value.
When contacts will submit the form, this value will automatically be updated to the contact field if contacts do not fill the form field with other values.
It is also possible to add this function with a hidden field.
In that case, add a new hidden field.
In the general tab, fill again the "default value" field and map it with a contact field.
Thus, when contacts will submit the form, they won't see this field but the contact field mapped to it will be updated with the value set (Yes in this example).
You can add several default values using the | separator.
value 1 | value 2