When a user wants to share a document or folder, they can get a link to it to send to others:
SharePoint Online supports three types of links: Anonymous access links (accessible by anyone), internal links (accessible only by users within your organization) and direct links (accessible only by users who already have permission to access the document). As a SharePoint admin, you may want to enable users to send anonymous access links (as these provide the smoothest experience) but you may not want it to be the default link when the user opens the Get a link dialog box. You can set the default type of link to something more restrictive, while still allowing users to select other types of links as needed.
Note: Note: Internal links are also referred to as Company wide links in other parts of SharePoint.
Change the default link type
Sign in to Office 365 with your work or school account.
Select the app launcher icon in the upper-left and choose Admin.
Tip: The Admin tile appears only to Office 365 administrators.
Go to the Office 365 admin center (above).
In the lower-left navigation, expand Admin and open the SharePoint admin center.
In the left-hand navigation, choose sharing.
Under Default link type choose the option you want to show by default when a user gets a link.