public Task ActivateDefaultToolAsync()

await QueuedTask.Run(() =>
{
  // Get all templates for the layer
  var templates = featureLayer.GetTemplates();
  if (templates.Count == 0)
    return;
  // Get the first template - alternatively get a specific template
  var template = templates.First();

  // Activate the default tool
  template.ActivateDefaultToolAsync();
});

await QueuedTask.Run(() =>
   {
     //hide all tools except line tool on a given feature layer
     var editTemplates = featureLayer.GetTemplates();
     var newCIMEditingTemplates = new List<CIMEditingTemplate>();

     foreach (var et in editTemplates)
     {
       //initialize template by activating default tool
       et.ActivateDefaultToolAsync();
       var cimEditTemplate = et.GetDefinition();
       //get the visible tools on this template
       var allTools = et.ToolIDs.ToList();
       //add the hidden tools on this template
       allTools.AddRange(cimEditTemplate.GetExcludedToolIDs().ToList());
       //hide all the tools then allow the line tool

       allTools.AddRange(cimEditTemplate.GetExcludedToolIDs().ToList());

       cimEditTemplate.SetExcludedToolIDs(allTools.ToArray());
       cimEditTemplate.AllowToolID("esri_editing_SketchLineTool");
       newCIMEditingTemplates.Add(cimEditTemplate);
     }
     //update the layer templates
     var layerDef = featureLayer.GetDefinition() as CIMFeatureLayer;
     // Set AutoGenerateFeatureTemplates to false for template changes to stick
     layerDef.AutoGenerateFeatureTemplates = false;
     layerDef.FeatureTemplates = newCIMEditingTemplates.ToArray();
     featureLayer.SetDefinition(layerDef);
   });