How should I include  or  inside ?
First, define a library in your theme that links to the font.

[THEME].libraries.yml:

fonts:
  css:
    theme:
      # Do not include http: or https.
      # Should start with a double slash.
      //fonts.googleapis.com/css?family=Source+Sans+Pro: { type: external }


This library is loaded by adding it to the theme's [THEME].info.yml file:

libraries:
  - [THEME]/fonts