Hi,
Refer this linkHow to use new Dynamic Default Value for Universe (UNX) Prompts feature
if you want to create the filters at the report level itself. As Mahaboob mentioned and as per this link write formula to derive the current month and year and then convert it to date y appending 18 to it.
If you want to do it at the universe level,
if your database is SQL server then
convert(varchar,18+""+datepart(day,getdate())+""+datepart(year,getdate()),110)
This is an example and you may need to follow different syntax based on your database