How do I insert a JavaScript variable into a string containing Jinja markup?

I have a javascript function that is trying to dynamically insert an anchor tag into a webpage, the href attr of which being a url for a flask route. This flask route accepts an argument username. I want to set username to the value of a javascript variable.

I have tried using template literals:

let username = 'user1';
let href = `{{ url_for("some.route", username=${username}) }}`
$("#elem").append('<a href=' + href + '></a>');

which throws a jinja2.exceptions.TemplateSyntaxError: unexpected char '$' error.

I've also tried string concatenation:

let username = 'user1';
let href = '{{ url_for("some.route", username=' + username + ')}}'
$("#elem").append('<a href=' + href + '></a>');

which results in the literal string + username + being passed to the route.

Any ideas on how to do this?