You need to add a Custom Field to Project Issues.

First, log in as Administrator and make sure the Technical Features are enabled. In the Settings menu:

• Go to Users, find and open "Administrator", and in the Access Rightstab checkTechnical Features`.

Second, create the custom field in the database:

• Go to Technical -> Database Structure, find and edit the "Project Issue" model
• At the bottom of the Fields tab, "Add an Item"
• Create the new field:
  • Name: x_url
  • Field Label: Hyperlink
  • Field Type: char
  • Size 200
  • Save & Close the "Create Fields" dialog
• "Save" the Project Issue changes

Finally, add the field to the form:

• Go to Technical -> User Interface, find and edit the "Project Issue Tracker Form"

• In the Architecture text, find and replace:

                  <group groups="base.group_user">
                      <field name="user_id"/>
  

with

                    <group groups="base.group_user">
                        <field name="x_url" widget="url"/>
                        <field name="user_id"/>

And it's done. Now go to Project Issues and you'll see the new field available.

For fields that are not URLs just remove the "widget=...". Just write: <field name="my_field_name"/>. See all attributes a field can have inn the official docs.