How to filter a lookup in a worksheet page

Magicman

Hi everyone. I'm very new to c/al coding and am struggling to set a filter on a lookup in a worksheet page. The lookup field is 'account no.' from the gen. journal line table. The table itself has a filter on this field which populates the lookup according to the entry in 'account type' and if type is 'Customer' all Customers are shown. I do not want to alter the filter within the table, only what the user sees in the page such that only customers that are not blocked are shown. I think this would need to be coded within the OnLookup-Trigger - is that right? How would i go about setting this filter?

Thanks for your time and any help you can provide.

M.