If the user navigate to Lists > Relationships > Contacts > Edit an existing Contact. Click Customise > Customise Form > Under Fields tab > Marketing subtab > the mandatory box for the field Lead Source is greyed out
Marking the Lead Source field as mandatory in the contact form is currently an Enhancement. This is filed under Issue #254970.
As an alternate solution, the user can do the following:
* Navigate to Customization > Workflow > Workflows > New.
* Enter a name for the workflow.
* Record Type = Contact
* Release Status = Released
* Set On Create = T. You can also set the On Update = T for existing records.
* Save the workflow.
* Click New State, enter a name for the State and save it.
* Click on the state and on New Action.
* Click Set Field Mandatory.
* Under Parameters select Field = Contact Lead Source.
* Check the Mandatory checkbox.
* Click Save.