We must use here Powershell Management tool for sharepoint online.
I posted here how to use it:Connecting to Sharepoint online site using SharePoint Online Management Shell
Manage external sharing for Office 365 Group site collections
By default, all SharePoint site collections that are part of an Office 365 Group have the sharing setting set to Allow external users who accept sharing invitations and sign in as authenticated users. To change this setting, you can use the Set-SPOSiteWindows PowerShell cmdlet to update the SharingCapability parameter.
Set-SPOSite -Identity https://contoso.sharepoint.com/sites/site1 -SharingCapability SharingOption
For SharingOption, use one of the following parameters:
|Disabled||Don’t allow sharing outside your organization.|
|ExistingExternalUserSharingOnly||Allow sharing only with the external users that already exist in your organization’s directory.|
|ExternalUserSharingOnly||Allow external users who accept sharing invitations and sign in as authenticated users.|
|ExternalUserAndGuestSharing||Allow sharing with all external users, and by using anonymous access links.|
So here we will use ExternalUserAndGuestSharing
Then powershelgl code will become:
Set-SPOSite -Identity https://contoso.sharepoint.com/sites/site1 -SharingCapability ExternalUserAndGuestSharing