Creation of Task and Event Custom Fields in Salesforce

Creation of Task and Event Custom Fields in Salesforce

Object Manager | Task or Event | Fields and Relationship tab | No New button to create new fields for such object

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

Steps:

Gear icon on the top right | Click on Setup | Click on Object Manager | Enter Activity in the Quick Find box and click the Activity label to open the object's setup | On the left side, click on Fields & Relationships | Click on New and proceed as usual

To differentiate between Task and Event custom fields, you can select appropriate page layouts (as shown in the visual below) during the field creation process. The field can be added to both Tasks and Events, or just one of the objects.

To edit such custom fields in future, go to Activity Custom Fields.

Visual:

https://www.youtube.com/watch?v=JErCdm58qkI

https://www.youtube.com/watch?v=8WahoTiwKjM

Reference: 

https://help.salesforce.com/s/articleView?id=000335522&type=1