To specify resource reservation at the job level, use bsub -R and include the resource usage section in the resource requirement (rusage) string.
For example:
bsub ‑R "rusage[tmp=30:duration=30:decay=1]" myjob
LSF reserves 30 MB of temp space for the job. As the job runs, the amount reserved will decrease at approximately 1 MB/minute such that the reserved amount is 0 after 30 minutes.