Interation between 2 webforms

[Anita C]

I have a webform- MainForm.aspx, which has a button amongst others on
clicking which a new window with a webform
opens(this.RegisterStartupScript("Browse Custom Reports Folder","<script
language='javascript'>window.open('ReportsFolderExplorer.aspx', '_blank',
'width=450,height=200,scrollbars=yes,resizable=yes');</script>");

In the webform - ReportsFolderExplorer.aspx the user can select a value from
a grid, which on a buttonclick needs to be passed back to populate
txtReportName in MainForm.aspx. The window which houses
ReportsFolderExplorer.aspx needs to be closed too.

How do I achieve the above.
Thanks in advance.