Making an input is the easy part but there are a lot more questions that need to be asked.

Of which country is your client a resident?

What kind of pension is that (e.g. employer-sponsored pension plan, individual pension account, social security, etc.)?

Which country's pension is that?

How was the distribution structured (e.g. lump-sum, annuity, etc.)?

How much tax was withheld?  Was that in line with any treaty provision that may apply?

What position was taken previously on the contributions and earnings?

Have foreign trust and PFIC considerations been assessed?