display method for all leads form showing the source campaign name

hello every one.

kindly i need to show campaign name to all leads form so i created new method on smmleadtable with the following code:

display smmCampaignName lookupsmmCampaignName()
{
    smmCampaignTable smmCampaignTable;
    select CampaignName from smmCampaignTable
    where smmCampaignTable.campaignId == this.sourceLinkedId();
    return smmCampaignTable.CampaignName;

}

but unfortunately there is an error in this.sourceLinkedId() part.

any help to set the relation between the sourceId and the campaign name

thank you in advance.