I found the way to do it. In sendgrid create an API key. Then in formr use as username "apikey" (those 6 letters, not the API key itself!) and in password write the password that was generated when creating the API key in sendgrid (it is a long sequence of characters). I have used port 587 and checked the box for TLS.
1. create API key in sendgrid (it will generate a password that will be used later)
2. in formr, go to the section to add email account and put
From (email): the email linked in sendgrid
From (name): your name
SMTP (host): smtp.sendgrid.net
SMTP (port): 587
TLS box: checked
password: password of the API key created in sendgrid
test box: checked
3. click on " Save Account"
I hope this small guideline helps people save time in the future!