Insert word in html tag with javascript

Ho folks I need to inject a word (required) inside a input HTML tag with javascript. I need to insert it based on a radiobutton setting. So button on: insert "required". Button off delete "required".

Any ideas?

<input type="number" class="form-api" name="ne5" id="num_elec_5" size="20" placeholder="Electriciteit excl.">

Has to become:

<input type="number" class="form-api" name="ne5" id="num_elec_5" size="20" placeholder="Electriciteit excl." required>