Riding with no bike lane

I’m fairly new to commuting and still learning best practices for riding with traffic. During my commute, I have to bike down a main street in my town which has no bike lanes. So I ride on the side of the street where cars park as close to the curb as possible. But a few times in my commute the parking lane turns into a turn lane for side streets.

My question is do I have to merge with the lane going straight or is it okay to remain in the turn lane to go straight? And if I remain in the turn lane how do I signal my intent to go straight?

I included a drawing to try to make my question a little easier to understand.

Thanks for any advice you can provide!