Q:

Fields are missing in Lightning Experience for Tasks/Log a Call/Events. Any workaround for this?

My question could be a duplicate of the below question that was not answered.
Email and Phone Task fields are missing in lightning experience 
Just added a custom field in Activity object using Salesforce classic. Standard Comments and the custom field meant to be available on Task layout. 
I switched back to Lightning Experience and tried to verify it. But, I have not found those two fields (the standard Comments field and the custom field which I have created). 
Please guide me on how to bring those two fields in Lightning Experience. Thanks. 

A:

I was able to add a custom field on Activity and add that on Task and Event page layouts while being in Lightning Experience. Have detailed out the scenarios with screenshots as how I was able to do this. All these steps were carried out in Lightning Experience (LEX).
Scenario 1 - While adding a field on Activity

From Setup --> Object Manager --> Activity: Added a custom field on Activity
While adding the field, selected the field to be available for both Task and Event layout
Verified on a Task/Event record, that the fields were available

Scenario 2 - When field is already added on Activity, but not available on the Task/Event layouts

Navigate to a Task/Event record
From Setup gear icon, click on Edit Object
When in Object Manager, go to the Page layout, add the field as required and it reflects on the record

