updatedhow to set default value in a form field got from another page(form)?
Drupal doesn't store the form state between forms, so you will need to do that yourself.

In this case I would use the session variable since you want to save it per user, but not permanently.

To do this you will need to add a custom submit handler to the form if you form is not created by you, this can be done with hook_form_alter

Then you can save the value you want to save in the session variable something like

 $_SESSION['my_value'] = $value;


should do the trick, then you can set the value in the form, either directly if you control it or via form alter.

That the form value is gotten from or is to be set to a field doesn't change things. The form field structure is a little more verbose, but the way to do it is pretty much the same.