To include an "Apply Now" button to the end of the job, check the "Apply through given instructions/notes" button.
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Then, in the text field revealed,  insert the following code:

<div class="l-inner"><div class="job-description">
<a class="btn btn-primary" href="https://riderlevettbucknall.recruitee.com/o/senior-quantity-surveyors-mrics" target="_blank">
<span class="btn btn-primary">APPLY NOW</span></a></div></div>         

For the href - highlighted in red; please insert the code provided by the client to their external system