How to insert for loop variable into output dataframe as a new column

[sten]

I have a list of variables(a,b,c) from df1 which i want to pass into a API call. When we pass each variable it gives out data which may have any number of rows depending on data.
Below is the code I am using:

df$list ### has variables (a,b,c)
output_df<- data.frame()

for(k in 1:length(df$list)) {
  
  res1z = GET(paste0("url1",df$list[[k]],"&StartDate=02-01-2023&EndDate=02-28-2023"))
  dataz = fromJSON(rawToChar(res1z$content))
  dataz <- as.data.frame(fromJSON(rawToChar(res1z$content)))
  # dataz$emp_code<- df$list[[k]]
  
  
  output_df <- rbind(output_dfz1,dataz)

The expected output i am trying to get is adding that variable column to the api output.When I try to insert it just inserts the variable a or b for just the first record.

SNo          OP2          OP3         OP4         OP5     Variable
1            5.1         3.5          1.4         0.2     a
2            4.9         3.0          1.4         0.2     a
3            4.7         3.2          1.3         0.2     a
4            4.6         3.1          1.5         0.2     a
5            5.0         3.6          1.4         0.2     a
1            5.1         3.5          1.4         0.2     b
2            4.9         3.0          1.4         0.2     b
3            4.7         3.2          1.3         0.2     b
4            4.6         3.1          1.5         0.2     b
5            5.0         3.6          1.4         0.2     b