I'd just talk to them about it. Sounds like you are at a startup, so the likely hood is you will still have to write marketing content- everyone wears many hat

I'd just talk to them about it. Sounds like you are at a startup, so the likely hood is you will still have to write marketing content- everyone wears many hats. But they could compensate you as a developer instead of a marketer. At my startup, I spend about 75% of my time working on sales because that is something we need to do, and I'm the best person to do it. As a coder at heart, I take an engineering approach to sales and use my coding skills to automate parts of the process.