Is Microsoft form require separate license? Can I share the survey link to my external users?

Microsoft Forms is included with all Office 365 Education suite licensing and for commercial customers it's part of the following plans:

Office 365 Business Essentials, Office 365 Business Premium, and Office 365 Enterprise E1, E3, and E5 plans. Microsoft Forms is also available to existing Office 365 Enterprise E4 customers who purchased E4 before its retirement.

See the https://support.office.com/en-US/article/Frequently-asked-questions-about-Microsoft-Forms-495c4242-6102-40a0-add8-df05ed6af61c for more details. You can share a Microsoft Form survey externally using the 'Anyone with the link can respond' option and then send the link to those external users you want to respond.