:setvar serverPri "serverA"
:setvar serverSec "serverB"
:setvar DBPri     "myDB"
:setvar folderBak "\\serverC\backup\"

:connect $(serverPri)
declare @backupfolder varchar(128)

select @backupfolder=pd.backup_share
from msdb.dbo.log_shipping_primary_secondaries ps
inner join msdb.dbo.log_shipping_primary_databases pd on ps.primary_id=pd.primary_id
where ps.secondary_server='$(serverSec)' and pd.primary_database='$(DBPri)'

print @backupfolder  -- this works like I want

:setvar folderBak "@backupfolder"  -- this doesn't work like I want
GO

:connect $(serverSec)
print @backupfolder  --this errors out because it's a different server