Create custom fields for Tasks or Events

Since Events and Tasks are both part of the Activities object, you will need to create your custom fields at the Activities level. 
  
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In Salesforce Classic

1. Go to Setup.
2. Enter Activities in the Quick Find box.
3. Click on Activity Custom Fields.
4. Click on New.  

In Lightning Experience

1. Click on the gear icon on the top right.
2. Click on Setup.
3. Click on Object Manager.
4. Enter Activity in the Quick Find box and click the Activity label to open the object's setup.
5. On the left side, click on Fields & Relationships.
6. Click on New.  

To differentiate between Task and Event custom fields, select the appropriate page layouts on step 4 of 4 during the field creation process. The field can be added to both Tasks and Events, or just one of the objects.

Once the field is created, you will still need to come back to Activity Custom Fields if you need to make any changes to custom Activity fields.