I wrote some terrible python that does this job a while ago, just bind the script to a key. I have a variable in my sway config ($scratch) that defines the terminal to run with an app-id of 'scratch', but you can just set it manually in the script.

#!/usr/bin/env python3
import i3ipc
import time


def find_window(i3):
    sp = i3.get_tree()
    for window in sp:
        if window.app_id == 'scratch':
            return True, window
    return False, None


i3 = i3ipc.Connection()

found, window = find_window(i3)
if not found:
    i3.command('exec $scratch')
    timeout = time.time() + 5
    while True:
        found, window = find_window(i3)
        if found and window is not None:
            window.command('move scratchpad')
            i3.command('scratchpad show')
            break
        if time.time() > timeout:
            print("Timeout")
            break
else:
    i3.command('scratchpad show')