You login form shouldn't "know" what called it - it should communicate with whatever "parent" form via events and a Result code when it closes just as all other controls do. To start "calling methods" in another form breaks OOPs!
If that isn't followed, then you get maintenance problems because the two forms are "tied together" and you have to consider very carefully what the affects of any change on one might have on the other.
So have a look at this:
Transferring information between two forms, Part 2: Child to Parent[
^] and think about what you are trying to communicate between the two forms.
And ... you do realise that settings files are plain text, don't you? They are just stored as XML data, so your login data is accessible and readable by anyone with physical access to the computer? That does rather make your security redundant ...