I custom build a site in Squarespace and bought a domain in Squarespace too. How do I connect them?

Lets say I custom build a site using Squarespace puma-puma-3spd.squarespace.com. (I subscribed for this)

I bought a domain in Squarespace, lets say 123.com (I bought this domain for one year.  On Squarespace,  it came with the build site where I can build a website.  I don't need it. All I want is, connect my custom build puma-puma-3spd.squarespace.com to link to 123.com) 

How do I connect 123.com to my build site puma-puma-3spd.squarespace.com? 

One way I tried was, I went to my workspace and setting -> domain and then choose the option connect the domain that I already have.  I entered the info and the option below was showed.  I saw some videos, there an option we can use the connect domain and drop down menu will show up and then choose the provider of the domain and easily connect.  Here, I'm not seeing the dropdown menu.  All grey out and the buttons are not clickable.  When I click the these instructions it's taking me to the domain page again.  I feel like I'm going through a loop. 

With this problem, you will need to contact Squarespace Customer Care.

Perhaps the previous owner of the domain name used it for an SS site, so it is still in the system, so you will need to contact SS Customer Care, so they can check and delete it.

I have this same problem....did you get it resolved?

I'm encountering this problem right now. What should I do?