Link to a #XXX in the menu?

Hello,

I'm ne at Grav CMS and it is a really good CMS.

But I have some questions.

1. On a Onepager Website, I like to work with "scroll to" #XXX. But I don't know how
2. How I become the #xxx than in the menu?

Lat question.
When I install a Plug in over the Admin Panel, how I can get this Plug in to my Website? How I have to insert that?

Many thanks.