Step 1. Go to Settings > Menu and select the menu you want to add the custom links into 

menu_1.png

Step 2. Add Custom links from the sidebar.

custom_link_1.png

Step 3. Click on the Settings icon and add in the anchor field the name of the block with #block-name, make sure to include the #. 

custom_link_2.png

The block name is the name you give to your block from the block advanced settings. 

custom_link_3.png