If your theme does not have that already, most likely you need to add some code.
I can link examples from Dawn and Horizon themes but you will probably have to adjust some code, maybe some variables are missing. But as a starting point, it should be OK.
Shopify doesn’t currently offer a fully native manual currency selector without using either Shopify Markets features or adding some custom code, especially when you’re working with a custom theme. In most cases, developers add a currency selector using Liquid, JavaScript, or a lightweight app that integrates with Shopify Markets to allow users to switch currencies manually alongside automatic country detection.
If you’re using any app then 99% of the case, shopify apps itself provide you the option to show the currency conversion.
Mostly they provide you the Sticky widget that will shows on your site. So, in this case you’ll need to enable this widget. You’ll find this setting in the app itself.
If you open to share the store url and app name, then it would be better to determine and provide you the exactly solution on how you can add it.
When you say ‘Shopify Markets’, I assume you mean the Markets section in the Admin panel? If so I have my markets, shipping, automatic country detection enabled. However I can’t see an manual currency selector.
Is there an app that you would recommend?
Or do you think using the Shopify Markets API and custom code is the best approach (My coding knowledge is minimal but I’m happy to learn).