How to ask for a salary increment in creative jobs where the KPIs may not be as clear as for example sales jobs?

First your have to figure out your "Why".

Are you offering more value than you're paid for?

Are you underpaid for your job scope?

Are you headhunted for a better pay?

Once you have that sorted out, then you go about figuring out the "how".

Could you be a bit specific on what creative jobs are you referring to? Graphic design? Marketing? I think it's all different!