Get parameter from URL and pass into a flow via lightning component.

flowcmp.cmp

<aura:component implements="lightning:isUrlAddressable,flexipage:availableForAllPageTypes,forceCommunity:availableForAllPageTypes,force:lightningQuickAction" access="global" >
	
            
    <aura:handler name="init" value="{!this}" action="{!c.init}"/>

    
    <lightning:flow aura:id="flowData" />
    
</aura:component>

flowcmpController.js

({
   init : function (component) {
       	
        // Find the component whose aura:id is "flowData"
       	const flow = component.find("flowData");
       	
		const params = location.search; //equates to ?uid=003V000000rHMuJIAW
       
       	// Part of the query => ?uid=003V000000rHMuJIAW to 003V000000rHMuJIAW
        const uid = params.substring(5, params.length)
       
        const inputVariables = [
            {
                name : "ContactId",
                type : "String",
                value: uid
            }
        ];
       
        // In the component whose aura:id is "flowData, start your flow
        // and initialize the account sObject variable. Reference the flow's
        // Unique Name.
        flow.startFlow("FLOW_API_NAME", inputVariables)       
    }
})