Adding Multiselect lookup for a custom detail in a custom section

Hi,

I want to add a Multiselect lookup for a custom detail in a custom section. I have created a custom section with the name of Applications and added a custom detail with the name of Bank Details. In this detail, I have added a lookup called Bank Documents. In this lookup added 3 values. So, I need a Multi Select lookup for that lookup field. Can anyone please guide me on this?