This knowledge article gives detail steps on how this can be accomplished.
Service Desk Management 17.3 and higher
Follow these steps to add a link
1. copy gobtn_role.htmpl file from $NX_ROOT\bopcfg\www\htmpl\default to $NX_ROOT\site\mods\www\htmpl\default IF $NX_ROOT\site\mods\www\htmpl\default does not have this file already
2. open gobtn_role.htmpl file in $NX_ROOT\site\mods\www\htmpl\default and change
html = "<table class='product_info_container'><tbody><tr><td>" + html;
to
html = "<table class='product_info_container'><tbody><tr><td>" + "<a href='https://www.google.com' target='_blank'>" + html +"</a>";
and save the change.
Note: here https://www.google.com is URL link example. you can change to the external link that you would need.
3. run these commands from command prompt
pdm_webcache -H
pdm_webcache -b
Note: you would need to repeat steps 1 to 3 on other Service Desk servers if you have multi server install.
Now clear your browser cache and test.
Please do this in QA/DEV install and test fully first before implementing on production install.