Need a solution to solve an issue on Odoo custom module

employee appraisal odoo 14 community custom module sends an email survey link to the appraisers emails, however, from the email that the appraiser receives, the link to the survey is not hyperlink or clickable. it just a string. the developer of the module did not use a email template to send the link but rather, a python code below was used. the solution i need is simply to solve the problem described above. attached is the module.
----code--
url = survey_id.get_start_url()
url = url
mail_content = "Dear " + reviewers.name + "," + "<br>Please fill out the following survey " \
"related to " + self.emp_id.name + "<br>Click here to access the survey.<br>" + \
str(url) + "<br>Post your response for the appraisal till : " \
+ str(self.appraisal_deadline)