Title: User Input to local variables then scripting to perform a find.
You can't, you have to use one method or another to allow the user to enter the values into fields such as the global fields that you are already using.
Here's an example, with global fields. If you want to use local fields you can adapt it with the variables as I have already described above, but I see no purpose to doing so:
Show Custom Dialog ["Please enter some values" ]
# In the previous step, Specify two input fields to link to MeasurementRegistration::g_gr and MeasurementRegistration::g_meas]
Enter Find Mode
Set Field [MeasurementRegistration::GRLev ; MeasurementRegistration::g_gr]
Set FIeld [MeasurementRegistration::AsmtCode ; MeasurementRegistration::g_meas]
Note: Show Custom Dialog is only one of three different ways to do this. It's easy to set up, but you can't format the input fields with value lists and you can't validate the input until after the user dismisses the custom dialog so it may not be flexible enough to work. The other approaches, however, still use this same basic script.