After logging-in, go to your Crosspost Settings screen. From there, click the "Get value from Google" button.

This will bring you to a Google screen, which will ask Tabulas to post data to your Blogger account. HIt "Grant Access":

After granting access, you'll notice that the "Blogger Auth Token" value is now set. The next step is identifying your blog ID value (we need both to make crossposting work).
Go to Blogger.com in your browser, and log-in. Click the "Edit posts" button for the blog you have already linked to. In the URL, you'll see a URL which looks like this:

Simply copy and paste everything after the equal sign (the longer number) and save this value in Tabulas.
You've now successfully linked Blogger with your Tabulas!