"reserve your spot" is correct and usable in written English. You can use it to suggest that someone should secure their place in a particular event or activity before everyone else does. For example, you could say, "Reserve your spot for the conference now before it fills up."
Register to reserve your spot!
To reserve your spot, click here.
Or, you can reserve your spot here.
RSVP here here to reserve your spot.
Register now to reserve your spot!
Tickets are $1 to reserve your spot.
Best to reserve your spots right now.
There are only a handful of tickets for the event left, so if you haven't already reserved your spot, now is the time to apply for tickets here.
There are only a handful of tickets for the event left, so if you haven't already reserved your spot, now is the time to apply for tickets here.
Don't forget to order plenty of sun block when reserving your spot on the Ark.
And La Repubblica, the Italian newspaper, said that reserving your spot was an "ancient" habit, which probably started just after World War II, as tourists began to pour in.
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com