Set an alias, e.g.:
Alias name: set_target
Pattern: ^t (.*)$
Field: target = matches[2]
matches[2] calls the capturing group specified in the regex pattern above, "(.*)". Capturing groups are specified by parentheses. matches[1] calls the entire pattern matched by the regex.
Just for organization's sake, I typically group variable declarations under a Group, maybe called "CF Declarations". So you can have all your aliases which declare variables named as "set_XXX" and all of them placed in CF Declarations.
Then set an alias, e.g.:
Alias name: alias_murdertarget
Pattern: ^kt$
Field: send("murder "..target)