How do i make [postusername] show [field1]?

[native son]

Hi, i'm helping run a private clan board and we're trying to get the "Thread Starter" and "Last Post" areas on the threads display and forums display lists to show who posted by rank instead of the username.

we've assigned ranks through custom profile field 1 (we replaced the Bio part with it) and made it so that the members can't change it. Now we're just trying to insert that [field1] into the "Thread Starter" and "Last Post" areas. i looked at the code and i think i'm replacing [postusername] but just replacing it with [field1] doesnt work

any ideas?

thanks in advance!