How to forecast with certain conditions

I have a doubt about forecasting analysis.

Let's say I have a "time" variable and a "budget" variable. I would like to build two model:

1. time series model where I forecast the budget for the next, for example, 6 months;
2. another time series model where I forecast the next 6 months (same as above) but where I consider a loss of budget due to another lockdown (due to pandemic) during the months of november and december.

For 1, I would apply a standard forecasting method, but I have a doubt about number 2, what would be the right method and packages to do such thing, if possible?

I would like to do that for a thesis project but I just started to scratch the surface of time series analysis.

$$y=b_0+b_1x+b_2D_{lockdown}$$
When lock down is imposed $$D_{lockdown}=1$$, else $$D_{lockdown}=0$$. The coefficient $$b_2$$ will give an estimation of how much the intercept of $$y$$ changes when lockdown is imposed.