How to load external javascript with payment gateway

Hi, I need your help !!
The need to include javascript file payment gateway to make payment through a form.
Join the following line in the src / index.html file but when compiling the app is not inserted the url and the form is not loaded.
The line income is as follows:

If anyone can guide me as are the steps to insert an external library is agradeceria much.

Thank you very much