How to automate drop down list allocation given specific entries ?

Alex, please watch this video as this is what you would like to happen

PHP Code: 

https://youtu.be/yH3zPULELAM 


. any questions or need help just message me. I got this to work in my sheets.

Hi Katie,

Yes not a bad idea !
Sorry I forgot to revert back. Still haven't gone live with new solution.
So at this stage, the only thing I'm missing is just to make sure that history isn't modified due to VlookUp function.

For example, in file attached,on sheet " supplier inv" in cell E6 I want to make sure that the entry is looked and won't be modifed.
Would you have any idea how to implement this ?

Thanks again !