Hi @arivz,
Welcome to the RStudio Community Forum.
This is a very trivial task in R (and one which you could easily have answered yourself by reading one of the many, free, on-line resources on learning R):
df <- data.frame(Customer = c(78,123,45,22,11),
Sales = c(1245, 1833, 776, 545, 1108))
df
#> Customer Sales
#> 1 78 1245
#> 2 123 1833
#> 3 45 776
#> 4 22 545
#> 5 11 1108
df$New_column <- paste("FRANKFURT", df$Customer, sep="_")
df
#> Customer Sales New_column
#> 1 78 1245 FRANKFURT_78
#> 2 123 1833 FRANKFURT_123
#> 3 45 776 FRANKFURT_45
#> 4 22 545 FRANKFURT_22
#> 5 11 1108 FRANKFURT_11
Created on 2021-06-05 by the reprex package (v2.0.0)