Where Does 1IM Pulls Selective List of Columns for Request Properties From?

I'm trying to build a custom form in web designer and need to load same/similar list of columns for request properties "ColumnName" column. When you go to manager and try to add a new request property, You can select a column from the drop down list, I want to create same dropdown in webdesigner but not sure where to source this data from in 1IM schema?

These columns are a subset of columns from PersonwantsOrg table but not sure where is this subset being defined.

I thought it might have been defined in column schema for permitted list of values but there is nothing.