Hi,
I have some screen shot it will helpful to you .
First spy submit button of your web form using html mode.
use navigation stage and find out option Invoke Java script and Insert Java Script Fragment
1. Insert Java Script Fragment on element.
2.Then Invoke Java Script on that Element .
View attachment 1558088347403.png
Fig 1- Basic Java Script Invocation Flow
View attachment 1558088398653.pngFig 2- Script Data item which contain java script code that we need to inject on element
Here on 3 rd line i have selected calendar date using java script of goIndigo.com website
View attachment 1558088452843.png
Fig 3-Insert Java script on html element
View attachment 1558088500265.png
fig 4- Invoke Java script on element .
Note- Java script code will changed according your requirement.
You can also search more about injecting Java script on Internet .
also there are several videos also present on you tube .
I hope you it will help full to you.
Best,
Sachin