Link Libor interest rate to LD Loan

See helptext for INTEREST.KEY on LD

INTEREST.KEY
Contains the reference number of the appropriate interest rate which will be used for interest computation purposes.
The key indicates the rate to be used for interest computation purposes on Loans and Deposit contracts. The key can take the form of an entry on the BASIC RATE TEXT (and consequently BASIC INTEREST) and PERIODIC INTEREST tables.

A rate change on the interest rate table will automatically update at the End Of Day run all contracts linked to the BASIC INTEREST and PERIODIC INTEREST tables.

Validation Rules
1-4 numeric characters. (Optional input.)
Mandatory input if the interest rate type is FLOATING or PERIODIC AUTOMATIC.

Prohibited for FIXED, PERIODIC MANUAL and PERIODIC STRAIGHT.
Prohibited for discounted contracts.
Prohibited for Accounts Receivables and Sundry contracts.
Prohibited for Commitment contracts.

So define a BASIC.INTEREST / BASIC.RATE.TEXT for Libor and use it.
The margin can be defined in INTEREST.SPREAD.