For the sake of anyone else who comes across this thread, you could also use two “Render Text and Store as Variable” steps to define the strings first. This would allow you to completely avoid the quote problem (and would be helpful if your string had both types of quotes).
For example, you could have the first Render Text and Store As Variable step configured to store this as custom.start:
"hostname": "
And the second one configured to store this as custom.end:
"
Then, in your rule, you could use this expression to find text between both strings: