This is essentially a lime juice based 1:1 simple syrup (by weight), with a little splash or spritz of lime bitters or tincture or lime-based limoncello.
1 part lime juice by weight
1 part sugar by weight
Any lime (preferred) or citrus tincture, infusion, extract, bitters or peel.
Combine lime juice and sugar and stir to combine. Gently warm if need be in order to assure the sugar dissolves completely. Once complete, combine with a small amount (1/100th of a part or less) of your chosen citrus essence add-on. Ensure you can taste it, but you need not add any more than that.