Adding dynamic values to a lookup

Hi Team,

i have a scenario where i have to 2 lookups in a section - 

lookup 1 - which as fixed set up values (value1, value2, value3...)

lookup 2- whose values are dynamic and should be fetched from external api based on value from lookup 1. 

for example: if value1 is chosen, it hits external api and gets set of values that should be populated to choose from.

Please guide me in implementing the same.

Thanks,

Gokul