If you have replaced your web3 provider with Fortmatic provider, nothing needs to be changed for web3 contract transactions to continue working. The following is an example on how to initiate a smart contract call.
// Initialize providerimport Fortmatic from 'fortmatic';import Web3 from 'web3';​const fm = new Fortmatic('YOUR_API_KEY');window.web3 = new Web3(fm.getProvider());​// Get your contract ABI from compiled smart contract jsonconst contractAbi = [{"constant": true,"inputs": [],"name": "name","outputs": [{"name": "","type": "string"}],"payable": false,"stateMutability": "view","type": "function"},{"constant": true,"inputs": [],"name": "getValue","outputs": [{"name": "","type": "string"}],"payable": false,"stateMutability": "view","type": "function"},{"constant": false,"inputs": [{"name": "_name","type": "string"}],"name": "setValue","outputs": [{"name": "","type": "string"}],"payable": false,"stateMutability": "nonpayable","type": "function"}];​// Create contract objectconst contract = web3.eth.contract(contractAbi);​// Instantiate contractconst contractInstance = contract.at('0x4efA9a82bFE112a9174325561bcdD1c68044724c');​// Get user account wallet address firstweb3.eth.getAccounts(function(error, accounts) {if (error) throw error;// Make the contract call.contractInstance.setValue.sendTransaction('Hello World', {from: accounts[0]}, function(error, txnHash) {if (error) throw error;console.log(txnHash);});});
// Initialize providerimport Fortmatic from 'fortmatic';import Web3 from 'web3';​const fm = new Fortmatic('YOUR_API_KEY');window.web3 = new Web3(fm.getProvider());​// Get your contract ABI from compiled smart contract jsonconst contractAbi = [{"constant": true,"inputs": [],"name": "name","outputs": [{"name": "","type": "string"}],"payable": false,"stateMutability": "view","type": "function"},{"constant": true,"inputs": [],"name": "getValue","outputs": [{"name": "","type": "string"}],"payable": false,"stateMutability": "view","type": "function"},{"constant": false,"inputs": [{"name": "_name","type": "string"}],"name": "setValue","outputs": [{"name": "","type": "string"}],"payable": false,"stateMutability": "nonpayable","type": "function"}];​// Create contract objectconst contractAddress = '0x4efA9a82bFE112a9174325561bcdD1c68044724c';​// Instantiate contractconst contract = new web3.eth.Contract(contractAbi, contractAddress);​// Get user account wallet address firstweb3.eth.getAccounts().then((accounts) => {// Make the contract call.contract.methods.setValue('Hello World',).send({from: accounts[0]}).once('transactionHash', (hash) => { console.log(hash); }).once('receipt', (receipt) => { console.log(receipt); });});