The Custom Actions "Next Gen Code" is designed to create tags that are single-sizes. So when there is a multi-size placement, you need to follow these steps:
Be sure the codes are included from the Excel Worksheet
Because the fields can only contain one set of codes for the other networks, just enter the first size listed in the AppNexus placement name. In this case, it's the "970x90" shown. Also note how the "Size" is "X" from AppNexus. That's because this placement is a "Sizeless" in AppNexus.
Example of the Codes entered:
Custom Actions for Mult-Size
Under the column "Custom" select "CSTM" and then write a note under the "Note" column. In this case, this placement is both a 9780x90 and 728x90
Check the Selection Box
Need to check this box to ensure the placement is added to the files we'll create next.
1. Click Generate AST and paste the file to a .TXT file.
2. Click Generate DFP Creative and paste to a .TXT file also.
We're going to open up both the "AST" and the "DFP" files to make manual edits.