Asking employer to pay my xmas bonus straight into my pension?

Every year I get a Christmas bonus of about £500-£750. This year I thought I might ask my employer to pay it straight into my pension instead. However, there’s some things I’m not sure about.

1. Will doing this mean I don’t have to pay tax, NI etc on my bonus? (At the time of contribution) So the full £500-£750 would go in my pension?
2. What does this mean for my employer? Does it cost them anything in fees to put my bonus straight into my pension?

For context, I work at a <10 person company with no internal HR. Speaking to HR costs my boss money, so I’d like to go to him with this request as informed as possible. He has probably never had this request from other employees before.

Info on me: I am a base rate tax payer, and my bonus will not put me anywhere close to a higher tax band. I don’t have student loans. My pension contributions, both mine and from my employer, are gov minimums.

If there’s anything else I need to add for context please let me know!