Enterprise Custom Fields includes custom formula and lookup tables. We can create CustomField programmatically either from CSOM or PSI. Here below is the example of creating CustomField using CSOM.

Microsoft.ProjectServer.Client.CustomFieldCreationInformation customFieldInfo = null;
try
{ 
	projectContext.Load(projectContext.LookupTables,
			lts => lts.Include(
								lt => lt.AppAlternateId,
								lt => lt.Id,
								lt => lt.Name,
								lt => lt.FieldType,
								lt => lt.SortOrder,
								lt => lt.Masks,
								lt => lt.Entries
							));
		projectContext.ExecuteQuery();
		
	  Dictionary<Guid, Microsoft.ProjectServer.Client.LookupTable> availableLookupTables = new Dictionary<Guid, Microsoft.ProjectServer.Client.LookupTable>();
		foreach (Microsoft.ProjectServer.Client.LookupTable lt in projectContext.LookupTables)
		{
			availableLookupTables.Add(lt.Id, lt);
		}

	#region CustomField Creation

	customFieldInfo = new Microsoft.ProjectServer.Client.CustomFieldCreationInformation();
	customFieldInfo.Description = "Test Description";
	customFieldInfo.EntityType = projectContext.EntityTypes.AssignmentEntity;
	customFieldInfo.FieldType = Microsoft.ProjectServer.Client.CustomFieldType.DATE;
	customFieldInfo.Formula = "Your custom field formula";
	customFieldInfo.Id = new Guid();
	customFieldInfo.IsEditableInVisibility = false;
	customFieldInfo.IsMultilineText = false;
	customFieldInfo.IsRequired =false;
	customFieldInfo.IsWorkflowControlled =false;
	customFieldInfo.LookupAllowMultiSelect = false;

	customFieldInfo.LookupDefaultValue = new Guid();

	Guid lookupTableId = new Guid("// Available look up Id");
	if (availableLookupTables.ContainsKey(lookupTableId))
		customFieldInfo.LookupTable = availableLookupTables[lookupTableId];

	customFieldInfo.Name ="Custom field Name";
	Microsoft.ProjectServer.Client.CustomField newCustomField = projectContext.CustomFields.Add(customFieldInfo);
	newCustomField.RollsDownToAssignments = false;
	newCustomField.RollupType = Microsoft.ProjectServer.Client.CustomFieldRollupType.Formula;

	projectContext.CustomFields.Update();
	projectContext.ExecuteQuery();
	#endregion
  
}

catch (Exception ex)
{
	//log exception
}

Hope this code will help you.Thanks