I just did it in Sierra 10.12.5/Safari 10.1.1 by dragging the FTM URL to the little dot grid at the left of my favorites bar.

Edit: Or this, from Safari Help...

Add a website to your Top Sites: Move the pointer over the Smart Search field, click and hold the One-Step Add button that appears at the left end of the field, then choose Top Sites from the menu.

You can also drag a URL or bookmark to the Top Sites page.

Edited by artie505 (05/24/17 03:54 AM)
The new Great Equalizer is the SEND button.

In Memory Of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire