Form action based on condition logic

I have booking form which contains source, destination, type of journey(onward or return), onward date and return date. I want my form should proceed based on journey type.

Example: if user prefer journey as onward – bot should ask only onward date After filing source, destination and journey type

if user prefer return journey – bot should ask both onward date and return date. After filing source, destination and journey type

I want condition to be execute after filing destination slot

How can I solve this