From Dynamcs 365, go to Advanced Settings.
In Advanced Settings, Customizations select Customize System. In the menu on the left, under “Entities”, look for the entity you want to personalize. Navigate to the form where the new field is needed.
Under “fields” press “New” to insert a new field. The name of the field must be specified. At this point Dynamics creates “its” name for your field. For example if you named it “Origin”, Dynamics creates the name “new_Origin”. I advise you to modify it with something more specific referring to the entity you are working on: for example “new_Origin_Lead”.
You can select the type of field. If you want to have a drop-down menu displayed with a choice you have to select “option set”. Here you can create your own set of options but we advise you not to; if you create your set here, it will only refer to this field in this entity and it will be impossible to bind this field to another similar field in a different entity. To do this you need to bind the field to an existing option set, ie a set of options that can be used in different entities. Dyamics already has its very complete set of options, which can be integrated. The customization mask is always used to manage the option sets; under the heading “Entities” you will find the heading “Option Sets”.
Map fields between different entities
You can map attributes between entities that have an entity relationship. This lets you set default values for a record that is created in the context of another record
Fields can be inherited by different entities during a transformation. To do this you need to do a mapping of the fields. In the customization mask, select the entity you want to work on and select the type of relationship (1: N – N: 1). Now select only the customizable relationships with the drop-down menu at the top. Double click on the relation whose mapping you want to modify. In the menu on the left select “Mapping”. Press on New and select the starting and destination fields.