library(tidyverse)
mtcars %>%
mutate(fill="1") %>%
ggplot(aes(x=mpg, y=disp, fill = fill)) +
geom_violin(aes(linetype = "pattern"),
key_glyph = draw_key_path)+
guides(fill = guide_legend(
override.aes = list(linetype = 0)))+
theme(legend.key =element_rect(fill=c("salmon")))+
scale_fill_manual(name = "fill legend",
values = 'salmon',
labels = c('fill here'))

I could not find a way to separate the legend.key.
If anyone else knows how to do this, please let me know.