See example for the element order in the vectors :
inpboxec.png
To get the menu, just use the following code:
vlabel<-c('matrix','combo')vcombo<-c('a','b','c','d')inp<-c('M')ans<-inpboxec(vlabel,vcombo,inp)
References
Lawrence M.F.,Verzani J., (2012) Programming Graphical User Interfaces in R,CRC Press Taylor & Francis Group