Add signature to signature set using iControl REST API

I had the same question. I create a custom signature set with all the recommended Log4J Signatures with this JSON body

{
    "name": "attack-signatures-log4j",
    "assignToPolicyByDefault": true,
    "defaultAlarm": true,
    "type": "manual",
    "isUserDefined":true,
    "accuracyValue": "high",
    "signatureReferences": [
        {
            "link": "https://localhost/mgmt/tm/asm/signatures/FzyRtmjEB-RDeYxdV22Wcg"
        },
        {
            "link": "https://localhost/mgmt/tm/asm/signatures/6GEkbeeViqG1i1bCMLXdOA"
        },
        {
            "link": "https://localhost/mgmt/tm/asm/signatures/TDtAdP40B84UaYXLhMgIew"
        },
        {
            "link": "https://localhost/mgmt/tm/asm/signatures/rYiJHZdAYt0dhXlbxFVXFQ"
        },
        {
            "link": "https://localhost/mgmt/tm/asm/signatures/tOd2EOQdCzetaf2U3fHdKg"
        },
        {
            "link": "https://localhost/mgmt/tm/asm/signatures/r1KtsZ7eOufX3tm3P1rhUA"
        },
        {
            "link": "https://localhost/mgmt/tm/asm/signatures/NQyazwn_x4RzMCHvqv4GBw"
        },
        {
            "link": "https://localhost/mgmt/tm/asm/signatures/T-E0ExygzH1O3DNShs3QtA"
        },
        {
            "link": "https://localhost/mgmt/tm/asm/signatures/MFZSx7kIuGwhZDl8Vq85gA"
        }
    ],
    "defaultLearn": false,
    "defaultBlock": true
}