Tier 4 dependent application IHS issue

I have got a CAS for Masters course. I had filled the Tier 4 (Student General) visa application form for myself and PBS dependent for my spouse and 2 kids. After completing my application I was directed to the IHS fee and paid the amount

But after filling up the dependent visa application form for my spouse and kids I was directed to IHS fee and there most of the fields have been auto populated and few details it has asked me to fill.

I had no issues with details of my depentant applicant but after that it is asking me to fill the expiry date of my visa under the section:

"You're joining or remaining with this person in the UK"
My question is ,
1. The dependent is in Germany and I the main applicant is also in Germany and we are applying for visa together.
2. Under this section it is asking me for the expiry date of the main applicant, but I have not applied for the visa yet how can I get the expiry date of the visa.
3. This is a mandatory field and not allowing me to go further
4. When I have entered a dummy expiry date (cousrse duration 1 year + 4 months) then IHS fee is calculated as normal.

I have selected the right Link to fill in the dependent visa application
( 
https://visas-immigration.service.gov.u ... 1582626522
)

I had no problem in filling up my details and payment has been done, I am the main applicant, but I am not able to proceed for my dependents can u please guide.