You can customize your meeting's virtual lobby preferences in either Outlook while scheduling the meeting, or in the Skype for Business client after the meeting has already started.
Outlook for Windows
Click the Calendar icon > New Skype Meeting > Meeting Options, and then click the radio button next to "A new meeting space". After choosing your preferences, click OK.
Skype for Business on Windows
Click Meet Now > More Options button > Skype Meeting Options. After choosing your preferences, click OK.
Jump to Option B: Skype for Business on Windows
Option A: Outlook for Windows
A1) In the lower left-hand corner of the screen, click the Calendar icon.
A2) In the upper left-hand corner of the screen, click New Skype Meeting.
A3) Click Meeting Options.
A4) Click the radio button next to "A new meeting space", and then choose your virtual lobby preferences. Then, click OK.
B1) Click Meet Now.
B2) Click the "More Options" menu in the bottom-right, and then click Skype Meeting Options.
B3) Choose your virtual lobby preferences, and then click OK.
Your virtual lobby settings will be applied to your Skype for Business meetings.