Greyout Service No field after entering service no

hello all,

in CJ20n , for services

after entering Service No and Quantity the Service No field should grey out(i.e end user should not be allowed to alter the service No)...

how can i achieve this..

i tried by by going to SPRO->Materials Management->External service management->Define screen layout..

i could not achieve by this way, is there any other way to find a solution for this problem..

thank you all..