Hi Murtza,
Elena's tips solved my issue for me (thanks Elena!), but to clarify:
I hyperlinked "Click here", and used the token {{member.webinar url:default=https://testlink.com}}
When saving the email, Marketo presented an alert saying that an invalid token was used, and replaced {{member.webinar url:default=https://testlink.com}} with {{member.webinar%20url:default=https://testlink.com}}.
When I went back into the email and deleted the "%20" and re-approved the email, it worked.
Thanks,
Lindsay