Add &tab=1 to the end of the URL of the NetCommunity page that contains the User Login part. 

Note: you must use the NetCommunity page id link, not the friendly URL address.  This will be in the format of https://yoursiteURL/page.aspx?pid=###.  You can find the page ID number by editing the page and looking at the URL in the browser address bar. The URL would be, for example,  https://yoursiteURL/page.aspx?pid=###&tab=1 where ### is the page ID.

If the login page has a friendly URL, such as https://yoursiteURL/login use ?tab=1.  The Site URL would then be https://yoursiteURL/login?tab=1