C# Mock unit test for Invitations.PostAsync() of GraphServiceClient

Question

I need to setup graphClient.Invitations.PostAsync() method. I need it to add some unit tests. But it looks like Invitations property cannot be mocked. Anywey, I am looking a way how to predefine a response.

My code is:

var options = new ClientSecretCredentialOptions
        {
            AuthorityHost = AzureAuthorityHosts.AzurePublicCloud,
        };

        var clientSecretCredential = new ClientSecretCredential(
            this._configuration.TenantId,
            this._configuration.ClientId,
            this._configuration.ClientSecret,
            options);

        var graphClient = new GraphServiceClient(clientSecretCredential);

        var invitation = new Invitation
        {
            InvitedUserEmailAddress = request.InvitedUserEmailAddress,
            InviteRedirectUrl = request.InviteRedirectUrl,
            InvitedUserType = SignInUserType.Member.ToString(),
            SendInvitationMessage = true
        };

        try
        {
            var result = await graphClient.Invitations.PostAsync(invitation);
            if (result == null)
            {
                // ...
            }


        }
        catch (HttpRequestException ex)
        {
            // ...
        }

How can I do this ?