I use Git in VS 2019:

I have an existing solution. When I right-click the solution, I do not see the option of adding it to source control:

You may wonder why the context menu has "Commit...". It is for a library used in this solution that is already under source control. This library is outside this solution's folder. I want to add every project in this existing solution to source control.
Could anyone offer a tip on this?
Hong