Squarespace as a blogging platform has been around since 2003. It was not until the 2011 release of Windows Live Writer that Live Writer really worked well with Squarespace. Configuring Live Writer to work with Squarespace is fairly simple.
First launch Live Writer and if have already configured a blog select “Add blog account…” from the blog accounts drop down.
In the first dialog select “Other services” and click Next.
Enter the URL for your blog on Squarespace blog. If you do not have a blog on Squarespace you need to go and create an account and then come back to Writer and enter the URL for your blog. Also enter your User name and password in this dialog. Then click Next. You will see that it is setting up your blog account. There is a slight chance that it might prompt you to do a temporary post to detect your blog theme but for Squarespace the themes are generally straight forward and you might not see the detection dialog.
On the final step you can enter a blog nickname that is show in the blog accounts drop down in Writer. You have the option to share your blog with those on Windows Live. Click Finish to complete the setup.
At this point you can now use Live Writer as the blog editor on Squarespace. Enjoy!