Can I fix the intercept of a categorical variable to 1

Interfaces brms
1

I have this model:

brm(bf(Y ~ intercept + x * slope * setting,
                 intercept ~ 1 + (1|id_f),
                 slope ~ 1 + (1|id_f),
                 setting ~ setting_c,
                 nl = TRUE))

setting_c is a factor with 4 levels.
I want the first level to have an effect of 1 (no effect when it is multiplied with slope), and the remaining 3 levels to be estimated.

Is it possible to specify this?

(Also, is it possible to specify this model as a linear model?)

2

After some thinking, I believe this the same mode, but specified linearly.

PPV ~ 1 + x:setting_c + (1 + x | id_f)

Now I just need to calculate the contrasts for setting_c.