If you have enabled the Quick Create Form option from “Calendar Configuration > Advance Settings”, you will get the CRM backend view (CRM Form view) to create the activity.
You will get a drop-down list of the activities that are enabled. Select the activity and click on the CREATE
After clicking on the “CREATE” button, you will be navigated to the CRM backend view (CRM Form) where you have to insert and select the details to add a new activity.
After inserting and selecting the required details, the activity will be created.
Note: The “Quick Create Form” setting is also provided at the user level. So, you can manage the Quick Create Form” options for the specific user from the “User Calendar Configuration”.