You want to use the vectorised version of if, ifselse(). I used NA to fill rows where EndUser !="B" but you may want to do something else.
By the way, notice that the operator for comparing two values is ==, not =
Df <- data.frame(EndUser = sample(c("A", "B", "C"),8, replace = TRUE ))
Df
#> EndUser
#> 1 C
#> 2 B
#> 3 A
#> 4 C
#> 5 A
#> 6 C
#> 7 B
#> 8 A
Df$EndUserB <- ifelse(Df$EndUser == "B", "B2", NA)
Df
#> EndUser EndUserB
#> 1 C <NA>
#> 2 B B2
#> 3 A <NA>
#> 4 C <NA>
#> 5 A <NA>
#> 6 C <NA>
#> 7 B B2
#> 8 A <NA>
Created on 2019-09-11 by the reprex package (v0.2.1)