Let's go.
1. Info:
Did You inspect this function module BAPI_PLANNEDORDER_GET_DETAIL ? It does not accept any ranges-tables / select-options as arguments to its parameters.
2. Assumption:
The only thing, which might be Your aim, based on the type of variable declaration at the beginning,
it_data type standard table of BAPI_PLDORDCOMP_E1.
makes me assume, that You might want to provide a list of components and therefore You think, You need a ranges table so that ONLY CERTAIN COMPONENTS are selected.
3. How-To:
If this is Your case, then You have to create select-options in Your selection screen, simple as this: (This might be a way, nevertheless, You should find the proper table, where all components of a plannedorder are stored, and use its "name"-matnr in the "FOR". ). This here simply shows, how it is done.
SELECT-OPTIONS: so_matnr FOR marc-matnr.
IF the program mourns, add the statement at the beginning, or in the top-include:
TABLES: "TheTableNameYouWantForFor".
If You add the SO_xxx to Your param's section, this will create implicitely a variable in the code, which serves as range-table.
In the debugger You can see something like this:

You now have to
- a) ANALYZE THIS RANGE-TABLE, in order to parse it properly
This is not easy, (there might be a function-module/method for that, but I do not know).
What You will basically have to do is, to extract those material-numbers, WHICH
are desired. This is not that obvious as it sounds, because a user also COULD
have entered SEVERAL RANGES / SEVERAL EXCLUSIONS / SEVERL SINGLE VALUES / SEVERAL INCLUSIONS.
The fields SIGN and OPTION are responsible for the combination mentioned above.
And SAP offers a lot.
This pic will show You, what this might mean.

- b) Once You parsed all stuff properly and really identified, WHICH MATERIALS SHOULD BE ALSO SELECTED (perhaps a second mediator table), You can
loop at the so_xxx-implicit-table or mediator-table into a struct, fetch the value, pass it to the structure of the argument-table of the bapi call and HOPE, that the materialnumber is enough for the BAPI-structure to deliver the materials (you should test this with se37 first). How to retrieve the materials from the implicit select-options-table?

But, from what I can see, and this would be the proper way, is to self-define a F4-callback, because You are already entering the key of the planned order.
This would mean, You would have a subset of all materials in the system, reduced to those being components of the planned order.
This would involve custom F4-Helps, custom F4-HelpExits, custom F4-Callbacks.
A little bit more effort.
In fact to much for a simple query like this.
4. Conclusion:
Check, what the BAPI does, reimplement it partially (check the selected tables, se37, and st05 prior to se37 will tell you), write the proper code Yourself ( or copy paste from the bapi), and You can use the matnr-range as it is posted above.