You have created your form templates, if not, more explanation here.
In Plezi, by default, all fields are optional, except the email field. The email is "THE" information to have in order to create a contact in Plezi. You will always find it when creating your form templates, with the asterisk indicating that it is mandatory.
To make another field mandatory (e.g. Last Name), follow these steps
In your form template, click on the edit button for your field;
Tick the "Mandatory" case;
Don't forget to save this change AND also to save you webform template
There you go !
Also, you can display an asterisk to indicate to your visitors that the field is mandatory, this is requested in the "Design" tab of your form. Check the corresponding box: