The AS Agreement ID is held against a learner's Employment Status record rather than against the entire learner. To enter the ID, go to the Employment Status tab in the Learner ILR Summary and select ... > Edit by the learner's current employment status record.
In the edit window, scroll down to find the AS Agreement ID field and enter it there. Remember to Save before leaving this window.
For more information, take a look at the full help pages for Employment Status and Creating an AS Cohort Export.
Keywords: Apprenticeship Services Agreement Identification, Agreement ID, Where is AS ID