using (ClientContext clientContext = new ClientContext("https://site-collection-url"))
{
    var userName = "admin@tenant.onmicrosoft.com"
    var password = "password";
    SecureString securePassword = new SecureString();
    foreach (char c in password.ToCharArray())
    {
        securePassword.AppendChar(c);
    }

    clientContext.AuthenticationMode = ClientAuthenticationMode.Default;
    clientContext.Credentials = new SharePointOnlineCredentials(userName, securePassword);

    // you can get the Guid of the extension
    // from the [Extension].manifest.json file's id property

    Guid spfxExtension_GlobalHeaderID = new Guid("<Guid of the extension>");
    string spfxExtName = "HelloWorldExtension";
    string spfxExtTitle = "HelloWorld Application Customizer Extension";

    string spfxExtDescription = "Adds HelloWorldExtension to the site";
    string spfxExtLocation = "ClientSideExtension.ApplicationCustomizer";
    string spfxExtProps = "";  // add properties if any, else remove this

    UserCustomAction userCustomAction = clientContext.Site.UserCustomActions.Add();
    userCustomAction.Name = spfxExtName;
    userCustomAction.Title = spfxExtTitle;
    userCustomAction.Description = spfxExtDescription;
    userCustomAction.Location = spfxExtLocation;
    userCustomAction.ClientSideComponentId = spfxExtension_GlobalHeaderID;
    userCustomAction.ClientSideComponentProperties = spfxExtProps;

    clientContext.Site.Context.ExecuteQuery();
}