Eu sugeriria primeiro fazer uma lista de motivos que justifiquem seu pedido, algo como: Why should I receive a raise?
Diretamente ou não, há muitas possibilidades. Seguem algumas opções:
I would like to discuss a pay rise. (straight to the point)
I would like to talk about a raise.
I was wondering if we could discuss that pay rise I deserve. (both formal and optimistic)
It is high time we discussed my pay rise. ("passou da hora"
- formal and a bit harsh)
Just out of curiosity the link below is about things you shouldn't say considering the presented context (pay rise):Pay Rise? - things you shouldn't say