For this, you can use the following code in your custom queries: 

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SELECT

'I-' || c.client_code || lpad(c8.cinv_numb, 4, '0')

FROM

clients c,
cinvoices c8

WHERE

c.client_id=c8.client_id

-------------------------------------------------

In this query:

'I-' is a string value which is used as the invoice prefix;

|| is a concatenation code which links several string values;

c.client_code is your client code;

lpad(c8.cinv_numb, 4, '0') is a code which will show your invoice number in the format "000X".