How to use sequential number in rest web service creation object?

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Hi,

i'm writing records with web service $create method, but now i have a problem with the sequential number.

How can I to have the max sequential number of one table that i have to use it to write in a table?

Ex. i'm writing in MFGHEADTRK table and i need to recover max MFGTRKNUM value (EX. AVP00000000015)

Can anyone help me?

thanks a lot...

Antonio