To enable shared channel collaboration with microsoft.com, you need to use the Teams admin center to configure the channel policy and the cross-tenant access settings. Here are the steps to follow:

Sign in to the Teams admin center with your Microsoft 365 username and password1.

In the left navigation, go to Teams > Teams update policies.

Select an existing policy or create a new one by clicking Add.

In the Update policy pane, under Shared channels, turn on the toggles for Allow shared channel creation and Allow external shared channel participation. This will allow users to create shared channels and participate in shared channels created by microsoft.com.

Click Save and then Close.

In the left navigation, go to Org-wide settings > External access.

Under Cross-tenant access settings, click Manage settings.

In the Cross-tenant access settings pane, under Shared channels, select Allow specific domains and then enter microsoft.com in the box below. This will allow users to invite Microsoft contacts to shared channels created by your company.

Click Save.

Note: You also need to make sure that microsoft.com has configured their cross-tenant access settings to allow shared channel collaboration with your domain. See Collaborate with external participants in a channel2 for more details.