In order to use the SEMrush API, you will need a paid subscription.
URL Profiler enables you to pull back SEMrush data in bulk at the domain level:
To access this and enter your API key, check the SEMrush tickbox under 'Domain Level Data'.
This brings up an API box at the top for you to enter your API key (see below for instructions on this) and a dropdown to select which national Google database to use.
SEMrush maintain 26 national databases, so if you are profiling British sites then you would probably choose the google.co.uk option.
If you are profiling URLs from many countries, you could let the tool automatically choose a database based on the domain’s TLD – defaults to the US database if TLD isn’t matched.
Whilst SEMrush do offer a free account, you need to pay to access their API. If you already have a paid subscription, simply head over to this page whilst logged in (if not, grab a free trial from the link above)
Simply grab the orange API key and paste it into the box on URL Profiler (see above).
If you don't subsribe to any SEMrush paid plans, they also offer API access from only $15.00 per month.